Welcome to the wild world of mower racing! You couldn't have picked a better group of folks to race with.
You can run any single cylinder engine, running the original engine that came with the chassis is not required. US rules say "Must be stock, complete, and not to exceed a factory rating of 33 CI." If you like the old flatheads, you can use those too.
I would strongly recommend going to letsmow.com and download yourself a copy of the 2015 rule book, a lot of your questions will be answered there. Yes, the 2015 season is coming to a close, and the new rule book comes out around February (I think), but you still have a close and accurate guideline, especially with IMOW. Their rules don't change much.
Good Luck!