Welcome to DKF! Nice pickup, and great work restoring that bad boy.
My cabs are on the list too (one TKG-2 red, one TKG-4 blue). Note, the original (first 3000 factory RS conversions) had serial numbers in the 10xxx range. TKG-3 cabs started at 01xxx, the first of which were red, and then transitioned to blue for the remainder of TKG-3, and all of TKG-4. I would be interested to see more pics of your cab. I'm sorry to be the bearer of potentially bad news, but it seems TKG-2 and TKG-3 parts have been pieced together for that guy. That said, the TKG-2 parts are highly desirable, and the TKG-3 red cabs are highly collectable. Would you mind posting a picture of the serial tag on the back of the cab and of the power supply, etc.?
I'm somewhat in the same boat with my TKG-2 cab - apparently all original parts except for the bezel and game board. The game board is a TKG-3-6...but, with original "ladder cheat code". Pretty close to original, but I'm still holding out that some day I will reunite the cab with a
TRS-2->TKG-2 4-board stack.
As for score submission, I think you're already on track with this, but your 4-boards stack, if it has original code (and I hope it does) - what is known as US SET 2 in MAME, a.k.a. "ladder cheat" code - is not acceptable for score submissions. That's where the 2-board stack comes in, and I'm guessing that's your intent anyways. Just thought it worth mentioning, to avoid a submission that can't be accepted.
Lastly, there is plenty of evidence that the TKG-2 cabs did not have art. Mine doesn't and has no signs that it ever did. Others in the same serial range with apparently original paint have no side art and no signs it was ever there. I've seen pictures online from the period, with a red DK and no side art. So, I'm just trying to say you don't have to feel obligated to put side art on it.