You should get one of those Oscar Controls joystick-restrictor plates, I've heard lots of good things about them, and they'll make your 8-way joysticks into 4-way with the turn of two screws. Another option would be to see if someone could compile you a version of mame, or you could compile a version of mame that had Tron's controls set to 8-way instead of 4-way, which would fix the problem. I don't know how to do this myself, but I remember a while back a guy writting about how he just made his mame emulator into 8-way controls on all the games, and it fixed this problem completely.