Gasoline airplane engines are a fair bit more expensive than a weed wacker or chainsaw engine. If you want to look at purpose built engines, look for DLE or DA gas engines. Ask or search the gasoline engines section at the bottom of the main forum page.
Edit: Added links
Gas Engines forum here on RCU: http://www.rcuniverse.com/forum/forumid_142/tt.htm
DLE 20cc, smallest gas engine DLE makes. http://www.towerhobbies.com/products.../dleg0020.html
DA (Desert Aircraft) 50cc (smallest they make) http://www.desertaircraft.com/engine...p?Page=DA-50-R
I searched for Homelite on Craigslist and found at least half a dozen Chainsaw or weed wackers for $50 or less in my area. These engines have a lot in common with Ryobi, Weed Eater (brand), etc. Other than a prop hub, there isnt much needed to make a chainsaw or wacker engine work for an airboat. Yank the case off, and pull the "beauty" crap off and you're ready to rock. They're cheap, and can be beefed up with port work and different ignition systems. They can also be set up with a glow plug and ran on gas/glow fuel to save weight by dumping the ignition system.
I actually have a Homelite 30cc brush cutter engine that I'd sell.