The starter, cam profiles, ignition timing, etc.

It is very difficult, to say the least, to reverse an engine. There have been a few race teams that have converted H22A's to spin "backwards" to work with normal racing transaxles, though.

It's not something Joe Average can do in his driveway, if that's what you are getting at. If you did reverse it, you would have five speeds of reverse and only one of forward. I don't know about you, but running 90 miles per hour backwards down the highway doesn't sound like much fun.