The Echo 50cc will make the best engine by far, but they are hard to find used. They come in a chain saw, (slightly harder conversion), or in back pack leaf blowers, (quite a bit easier conversion). Try engine repair shops; that's where I get 95% of my engines.

The Poulan 46cc came in chain saws, and can sometimes be found "reconditioned". Try a google search for reconditioned chainsaws or lawn/outdoor equipment.

AV8TOR