Sounds familiar; I had thing magnet not working all the time (replaced magnet and founds wires were broken internally), thing opto not working all the time (again another broken wire internally leading to opto), ball trough switch randomly working which gave phantom drains during play and various other crap.
My game is in pretty mint condition and the previous owner had babied it for 15 years so I was really annoyed too - but it did travel 800 km to get to my house so I expected a few problems. The machine is now 3 months old here…
My advice is like that mentioned above, especially having two people working on it helps. I’m 6ft4” so reaching in isn’t a drama but putting it low to the ground makes sense if you struggle to reach. Don’t be afraid to get the multimeter and soldering iron out and try to isolate one problem at a time. You must source some manuals if you haven’t got one already. Parts are cheap, if in doubt replace them.
Once you nail one of the problems you’ll feel like a champ and you will be more confident to move to the next. I suggest start with the thing magnet fix first as it really is just removing the thing hand and testing the continuity of wires / fuses.
I like tinkering so for me it has been a fun adventure seeing how a 90’s machine was built - but worst case scenario - don’t sweat it - find a pinball repair guy and preferably get them to fix it in front of you if possible. I know it costs more money but you’ll have the knowledge and a working machine without stress, and as I get older this takes priority!
Good luck and let us know how you go - lucky we have these types of forums and helpful advice to follow - I couldn’t imagine how lost I would be back in the day without the internet and Pinside …!!