Hi
You can check
http://www.geocities.com/CapeCanaver...216/webww.html
http://www.geocities.com/CapeCanaver...6/geobook.html
or put in Yahoo search "Joe Guzzardi how to teach Ryobi" This is the easiest description of convert an engine I have seen.
I dont know your skills at mechanic but you can also bought pieces.
After it go to "rcfaq".
You can go to the local repair shop and dig in the dump to see engines, if you have luck with the owner you can buy for a couple of dollars good chainsaw engines, BUT before you buy take number and maker and go to RCuniverse to check for conversion posibility.
Good luck
Jose