Unless you are going to operate it, or this has some sentimental value to you, my advice would be to put on on free play and be done with it.

You said you are a newbie. This is a phase that most newbies (myself included) go through. Want the machines all complete, down to the coin mechs. Want to drop the quarter - to get that arcade feeling etc., etc. All that flies out the window in short order. Coin operation is a PIA on a good day.

My prediction is that if you stay with it, your Big Deal will be sold in a few years anyway. From the looks of that door, it probably isn't a HOU collectors quality thing. Just concentrate on getting it to run real solid. Getting the pops to pop well and the flippers to really flip. Clean it up nice. Get the score reels dialed in etc.

By the way, back in the day, when operators were unloading machines to the home market, some would strip out the wiring so that the machine wouldn't find it's way into a competitor's hands and be used as competition.

So...you likely have to figure out the hack and unhack it. Really, save yourself the aggravation. Just go the free play route. I have a game like this. It is a keeper and my designated "holy grail". I have no desire to try to put the coin mechs and associated wiring back in that game.