Depends on how much ya wanna spend too... an old MTB is an excellent donor bike, but there's also nothing wrong with going with a Monark springer fork, or a good tripletree type fork, or if money's not a problem, a downhill fork would suit that thing really well.
You definitely want to use a disk brake up front if you can so try to find a donor that already has it, it can be switched over later, but it's much easier to start with it.
There's a guy in here that makes custom solid seat posts for a decent price so if you want a good layback seat post that don't need to be braced up, that's another option.