If the game is a hack, go wild. Mix and match. As long as it has bits of Into the Odd in it, I'd say it's fine.
For content, try to keep within the basic rules (which are pretty lean already). That means auto-hit damage, three stats, failed careers, etc. That said... Who cares? Do what you like. Chris wrote a blogpost about what are essentially classes the other day, and the game was still the same otherwise.
Basically just absorb the rules and twist however you like. Maze Rats was based on Into The Odd, but became something completely different in the end. I'm pretty sure Ben Milton still considers it a hack, though.
Have fun!