Most of the 22 to 30cc guys require a bit of cutting, grinding, etc. to get them ready. People ask me how I do it, and I say "Well, I just cut and grind away everything that doesn't look like a model airplane engine.... "

Anyway, a nice engine close to your size range, but easy to convert would be more like the Homelite 33cc engine out of the "Bandit" chainsaws. Easy to convert, and they work really well. Another would be the Poulan 33cc. Weight will probably be the same, if not less than the 25/30cc engines that are a bit more work to convert.

Good luck, it's a blast,
AV8TOR