Watch eBay for the controls... They usually end up fairly cheap with no bidders - the shipping's usually the killer. If you don't need the bars themselves, you might be able to talk the seller into stripping them and shipping the smaller bits - should save some money that way. Brake / accel cables seem to be the worst parts to come by - if you don't need those, you should be able to get the rest pretty easily.
I'm just finishing up resto-MAMEing a Hang-On and have two boardsets that are supposed to be working (per the previous owner), though I had no opportunity to test them as the monitor in my machine was hosed (which is why I ended up going the MAME route - got ahold of some cheap 20" PC monitors). One was in the machine, the other was with a box of parts I got with the thing when I bought it. If you want them, I'd be willing to let them go cheap - they're just taking up space in my closet right now.