Ideally you want a WR500 motor, center exhaust, 5 speed tranny and lighting coil. the newer model 490s are center exh. but most of the yz, are 4 speed. The IT had the 5 speed tranny. the center exh.port is much easier to get and exhoust setup and line the engine up right. The actuall install is easy and if you have fab stuff can be done in an afternoon. Your girlfriend is going to love the bike because it will vibrate like a muther. The kickstarter can be heated with a torch and bent a little to clear everything.
You will also want to have the head done. and of the long standing 2 stroke porting companies should have the specks, it needs to be done or the motors like to detonate. I was a very common and nec. mod when the bikes were popular.