Don,
Check out this website for inexpensive gas conversion engines and information on interference issues. He has an 18cc and a 21cc on ebay right now for just over $100 each. I'm thinking the 18cc at about 3 lbs might be a great power plant. Also, on his site, there is an article about a 1/4 scale replica of the Merlin engine used in the P51 and Spitfire fighters. Pretty amazing.

www.indianatinygasengines.com