Quoted from cosmokramer:
I know some pinsiders like to weld a hole like this up and grind it down, but thats not how I do it. Quite a few restorations ago I had to deal with a similar problem with a pinbot coin door. I know some of you may frown upon this solution, but I like it. If you acquire a game and notice this coin door repair, you know you have a genuine cosmo restored game.
More on that in a bit...
I used epoxy to attach a quarter minted in 1990, the same year as Funhouse. Then filled the hole with bondo.