Go for the Super Tigre G34's. Good power and much less expensive than the OS 25. And since they are ringed engines, they will last longer than an ABC or ABN, will be more tolerant of a lean run, and much less expensive to rebuild (a ring versus a P/L assembly). The only downside with them as being a steel cylinder engine, you have to use a fuel with castor content or be religious with after-run oil.