Mike:

If it's a rotary valve, pretty much forget it. If it's a reed valve, or a "Three port" (piston controlled intake) it's simple. You don't have to do anything with them. The mechanical parts don't care which way they turn. Only problem is ignition timing. And that's not hard.

If you plan to use the flywheel magneto, check the coil position relative to TDC and the magnets, cut a new keyway in the flywheel to offset the magnets an equal amount on the opposite side of the coil at TDC. You're finished.

The exception here is the "Fuji ATM." For that to work you have to turn the magneto core around also - it uses the air gap on the core extension to get its "Automatic" timing advance. But this is only with the Fuji engines.

If using an electronic ignition system do the same flywheel trick with the pickup on the crankshaft.

Some of the spring starters can be reversed also, making the cconversion look like a "Factory" job.

Counter-rotating engines are the top.
. But where do you find a backwards prop?

Bill.

PS: If your engine has a rotary valve, some are easy, some are a pain. Come back with particulars, or any questions. wr