Aaaand done!

I cheated.

I finally found a pretty decent low-poly model in a suitable format for converting to MD3 with minimal hassle. It wasn't a Space Invaders machine though, so I had to remodel it a bit, make a new skin and tweak a few things. I guess I could probably have made a model from scratch myself but I'm not great at that and the tools that I am most familiar with are pretty basic. So using another model as a base seemed the best way to go.
It's maybe not historically accurate but it has the right kind of feel (there were loads of variations of cabinets back in the day anyway). I'm just using it as a bit of an Easter egg. So it doesn't really matter. I'm not making "Super Realistic Late 70s Similator".
I've animated the screen (just the invaders going between their two frames of animation) and it makes a suitable sound when it is used. Shame it can't actually play the game. That would be very cool.