The following is a copy and paste from the conversion engine discussion forum
Original: Av8tor1977
The Stihl engines are good ones for sure, but in general harder to find used than the other brands. Echo is another excellent engine, and somewhat more commonly found. They are from 21 to 25cc in the brush cutter/weedeater sizes.
Other good choices are Homelite 25/30cc engines, and the Ryobi 30/31cc. These make good airplane engines, though they are a bit heavier and not as powerful as the Echo engines.
All in all, my favorites are the Echo engines. Two examples:
One of my Echo engines is all "hopped up" with porting mods, etc., but it turns an APC 16 x 8 prop at 9400 rpms, weighs only 2 lbs. 9 oz. complete, and creates over 15 lbs. of static thrust.
Another one, that only has a slightly larger carb and a modified free breathing muffler turns a Top Flite 18 x 6 prop at 7500 rpms and creates nearly 15 lbs. of static thrust at a much lower pitch speed. The "hot rod" engine pulls a 72" wingspan Katana around like a rocket, and the milder Echo flies a 60" wingspan Fokker Triplane just beautifully.
Once again, this is a generality, but I have found a good airplane size for the weedeater engines to be from a low of 1000 square inches of wing area and 9 to 10 lbs. to a high of around 1200 inches and maybe 15 lbs. (There are exceptions of course.)
If you have welding and machining abilities, you can make your own mufflers and prop hubs. Most of my mufflers are just "gutted" and modified stock mufflers. You will have to decide if you want to keep the magneto, or convert to electronic ignition. I nearly always convert to electronic ignition for lighter weight, easier starting, and somewhat more power.
Sources for prop hubs, parts, etc.:
www.lambertsrc.com
www.wackerengines.com
www.carrprecision.com
www.southeastrcengines.com
www.valleyviewrc.com
www.milehighrc.com
(The last three are for ignitions.) Here is some other "food for thought" about ignition systems: http://www.rcuniverse.com/forum/m_80...tm.htm#8009384
Good luck. It is a fun (and cheap) hobby converting engines.
AV8TOR