Hi guys Just wanted to share my experiences going the conversion route. Inspired by the engine conversion section on RCU I picked up 'troy built' 25cc trimmer/weedwhacker engine on ebay for as little as 35 usd. Cut off all the excess metal - lightned it as much as I could, got a prop hub and EI for it. Gutted the stock muffler - its crimped together so I opened it and took out all the internal stuffing -wire mesh spark arrestor etc. Changed to a larger walbro carb. All up weight was around 1300 grams or so. Top rpm on an '18x6 apc w' was abt 6400 rpm
I have been using this engine on a 1/4th scale giles for about a year and a half. Performance may be described as 'sports aerobatic' - no unlimited vertical and definitely no 3d.
But the great thing abt a conversion is that it is always a work in progress. So after a rough landing/stall in a head wind, which broke the landing gear I decided to pull the engine and make some 'improvements' 
I noticed that more weight could come off the prop hub So I took it to a local machine/car repair shop and asked him to turn it down on a lathe. Cut some of the crank off that was extending behind the case. Now it weighs in at jsut over 1kg or 2.2 lbs
I also 'blueprinted' the engine to find out my exhaust and intake timings. These were rather conservative so I took a small file and increased the port size - top of the exhaust port and bottom of the intake port until my timing was 155 degrees on the exhaust and 135 on the intake. I also got rid of the head gasket to increase compression. Resealed using a 'liquid gasket maker'
well yesterday I ran it with some apprehension - but the thing seems to have a lot more power and pulls hard on an apc 17x6 I need to try some different, lighter wooden props on it. Without a tach (which is high on my shopping list now) its not possible to know the increase in rpms
guess when I go to the field Ill see if anyone can tack it for me - but the proof of the porting is in the flying!
Hope this might help someone get into gassers really cheaply. Besides I get a lot of satisfaction in seeing my grass cutter fly!         
       

We (me and my brother) are just getting into gassers

So this kind of a conversion, what would be the total cost.  The prop hub and EI (CDI unit?) were procured separately you said. And the Wallbro carb was also extra.  Wondering if the effort is worth the savings ? Of course the DIY satisfaction is precious !

Please post some pictures (before and after, if you have them).

hi Anwar, The EI is an Rcexl unit I bought from hobbycity for about 35 usd and the prop hub was for 18 usd on ebay. If you look in the engine parts section in hobbycity they have lots of mufflers, EI units and carbs quite cheap.
Walbro carb was 9 usd for three!  - again on ebay. So for a little over 100 usd I have a usable engine - maybe not for all out performance but would definitely work on many scale and sports models and for decent sports aerobatics - for example I can do KE and rolling circles but no KE loop or rolling loop. 
I did this conversion while studying in the US - I couldnt fly much so I was just doing these for fun in my spare time. (I have also done a poulan 42 cc chainsaw engine which I havent flown yet) In india getting the parts made might be even cheaper -  because labor is (sadly) so cheap - When I took the magneto out I got the hub (that holds the magnet) made for about 300 rs, and it took him 1/2 hr to do it. He matched the taper on the crank perfectly - so much so that I did not need to use the key - its a friction fit , followed by the prop hub which is screwed on tight (with threadlock).
I will post pics - my card reader is malfunctioning - so just need to get a new one.           

So the figure "little over $100" is excluding all shipping charges, right ?  The RCGF engines are also available at similar but slightly higher cost.

Again, the DIY part is where the real glory is !

PS: Please do post some of your flying videos also

No its inclusive of shipping. But yes its true -with the Chinese engines the cost has come down a lot - almost to the point where the conversion doesnt make economic sense any more  but I think it might still have some benefits - not in terms of power, unless you really hop up the engine, but in terms of longevity - these things are built to take a lot more abuse than any cheap chinese gasser. Our rc application is relatively less demanding.
Also in the next larger displacement - like 30 -50cc I think its still more economical than a chinese engine - you can still get a chainsaw engine for about 70 -100 usd and convert it for maybe 150 for good scale power. (closest is the crrc pro 40cc kit for about 200)
Also People are converting 4 stroke engines for that scale sound- and there are'nt that many commercially available ones (apart from saito and that aint cheap!) 
 
Dont have a vid camera but if someone brings one to the field will post. I just started flying with the guys at the Bhondsi field and I saw someone posted a short vid of the days flying -with diff models - that might be the more interesting way to go!!       

Here is a very informative page for hopping up a weedie. 
http://testfly.0catch.com/homelite/enginemod/cylindermod.html

couple of pics. I recently 'adapted' a large glow muffler for use on the engine - a little lighter than the original gutted one but could be made even lighter. Might get around to it one of these days    

Do they vibrate more than regular (purpose designed/built) gas engines ?  The mount looks like something is sandwiched between two ply sheets.

No its fairly smooth. Smoother than the MT 57 which is a brute for power but vibrates at just above idle. The mount is just cheap regular ply glued with araldite - nothing fancy here! The stand offs I made by cutting off the handles off some artist etching tools - since I couldnt find dowels anywhere. I remember it was a real pain to drill holes freehand and took a few attempts - they were still off center but I enlaged them so that they were usable. Later after a crash I rebuilt the front end fire wall etc. I added some thickeness to it and found that the prop wont clear the cowl - so added the plywood squares.