How complicated is repairing a machine like this?
You gotta find one first.
Depending on what needs to be repaired, it could be easy or a royal PITA. Plan on it being a royal PITA. So gather up your patience, then exploit the sources and expertise such as here or at the other sites mentioned. When you get frustrated, set the project aside, and learn as much as you can about the hobby in general.
For that kind of game, I guess I would start reading the pinball repair sites to see what kind of work goes into a machine like that. As much as I detest to say it, PBJ seems to have pretty useful advice in that area.
I'm on my, "pissed off, I'm going to take a vacation from this crazy ---steaming pile of meadow muffin---," stage. That will be after I finish jamming said project PCB down the throat of the nearest moron that asks me, "what are you working on?" while using my ancient, out of date, ---fouled up beyond all recognition--- up laptop to explore the stretchability of said moron's rectum.