We finally have a working prototype.
The trigger fires, and either top button centers the tank. 2-1/4" trackball on the left works like a mouse, I will probably reverse the Y-axis, which I think is more intuitive for aiming. Player 1 and 2 buttons, plus Continue and New Game buttons on the right.
Old Defender cabinet was a Mortal Kombat conversion when I got it. I will try a few other cabinets styles in the future. This one will be running at my TRON after-party in December.
I was able to get the Space Paranoids game to reside on the hard drive so this machine no longer needs internet access to be playable.