>>TILT<<
The first thing that strikes me is the ambidextrious agenda of the original CP. Not sure I've seen this type of design before.
Any reason you're using the original Main Event CP instead of starting from scratch? Will all the holes line up? Or are you just using the design? Seems like more trouble than it's worth to use the original board.
I couldn't imaging not having a trackball on my CP, just my opinion. I think I use it more than any other control. Spinner is iffy, but I'm glad I did it for Tempest (one of my favorites) and some driving games (not a great driving solution but better than the joystick IMO).
Looking at your design, I'd think the location of the dedicated 4 way could be a problem with the other buttons in front of it, but I'm not sure since I don't have one on my CP. But, my spinner is 'behind' everything else on my CP, and I know my arms/wrists/hands like to rest on the CP when using the spinner. Not sure how having buttons in front of the joystick would work, but I don't think I'd do it. Others that have done this will have a better idea of how practical this is.
Other considerations - even tho you only plan to play this, that, and the other game, you never know if you'll run into some game that requires more buttons or the 'traditional' SF layout. I'm a classic 80's game fan, but still like to play a bunch of other games, some which I never heard of before finding MAME that require more than 1 or 2 buttons. Also my kids and other family & friends seem to like the fighting games, so having the SF arragement has worked well without sacrificing my fondess for the oldies that only require a jump or fire button to play.
Bottom line is YOU have to live with it, so make it the way YOU want it! Good luck!