I managed to get back into working on these this summer. The control panel has been re-found, sanded and mounted into the carnival cabinet. I've an lcd monitor sitting in it now, although i'm not completely happy with it. Its what I currently have to work with. I ended up going with a 4-way stick and 2 fire buttons even though my test box had an 8-way and 3 buttons. The left-handed setup was a bit odd to get used to. There is still quite a bit of work to do on it.
I never did get that spinner hack working. However, i did manage to get a mouse hack on the Happ trackball functional this past weekend for the white cabinet. The first mouse worked until i tried fixing a solder joint and made a stupid mistake. I attempted to resolder one of the pins while it was still plugged in. No damage to the computer, but the mouse died. Luckily i had a belkin mouse and redid it a second time. I went ahead and added some wiring harness to the mouse buttons while i was at it. There is always room/need for more buttons. I don't know where i'm going to put the actual buttons though. Since they are buttons for the mouse, i'm thinking about putting them above and 1 on each side of the trackball mounting plate.
I havn't decided if i'm going to re-visit making a spinner. There is a rather nifty looking model file on Thingiverse for an arcade spinner that supposedly fits into a standard button hole. I actually downloaded one of the files on there to get button nuts. I printed out half a dozen to replace some of the metal ones I have.