I've never embarked on a restoration myself, so I'm not even sure where to start.
it really depends how deep you want to go, there are kind of 3 levels
1. getting everything running and working but not worrying about looks
2. doing #1 above but also some minor cosmetic restoration (this is what I tend to do personally)
3. a full tear down and rebuild
If this is your first one I'd recommend avoiding #3 for now and take a more phased approach.
0. ACCESS
I'm putting this here because you mentioned keys. I think on Candy Cabs in particular, original locks for all doors are important, the exception being the coin box (since 99.9% of the time it's different than the other locks and missing since most operators didn't want a universal key on the cash box). If you have original locks but don't have original keys. GET ORIGINAL KEYS, if the locks are not original still see if you can get the keys, or learn how to pick locks (it's actually easier than you think, especially the cheap locks on most cabs). Sometimes if you can access one door you can get to the back of another door and unbolt the tank on the lock to get it open. Drilling should be your absolute, there is not possibly any other way, last resort option. It's expensive: a good drill bit to cut through a hardened lock isn't cheap and will likely destroy the bit but the end of it, replacement locks, especially original locks are not cheap usually costing anywhere from $75 - $200 for a full set, depending on the rarity of the model. And you end up with metal shavings all over your cab that needs to be cleaned up. So yeah I recommend avoiding drilling if there is any other way possible.
1. PHOTOGRAPH
Start by taking an obscene amount of pictures of every little thing on the cab. that way if you take anything apart or rewire anything you'll have a reference to how it was before. which connector goes where, which bolt goes where, how were things oriented when they were installed etc.
it's also nice to have a great collection of "before" pictures to compare once you're finished.
2. DEEP CLEAN
clean everything as best you can, not only will this have a big impact in how the cab looks and feels but it gives you a better idea of what you're working with (dirt can often hide flaws), and it means you're not going to get filthy every time you work on it going forward.
You don't necessarily need to take anything apart to do this, but some minor disassembly might help in certain areas. I personally like to completely disassemble the control panel as that usually gets the most dirt of any part of a cab since that's where everyone touches it. If you're planning on replacing all or some of the panel this can flow into the next step
3. Hunt for parts/ make it COMPLETE
if there are missing parts start hunting for original replacements, this might take a while so it's good to get started documenting what's needed and hunting early. Don't be tempted to go with "good-enough" non-original replacements for stuff. I would avoid anything cosmetic right now, especially recommend AGAINST jumping straight to brand new repro side art/stickers This is very expensive and IMO might look less beat up but will actually devalue the cab if it's not flawless. Save this as the last thing you do. Here's where you need to focus on the actual electrical and mechanical missing parts of the cab. Not the art-work.
4. MAKE IT FULLY FUNCTIONAL
Make sure the monitor is fully working and tuned, take it out and give it a recap if necessary
Make sure the power supply is fully working and stable, take it out and give it a recap if necessary
Make sure that the marquee light works, replace the bulb and or starter if necessary
Check the speakers to see if they're dry rotted, rebuild them or research replacements if necessary
Determine if you want new buttons and joysticks on your control panel or to fully disassemble and rebuild the ones you have. Buttons and sticks are one of the very few parts of an old cabinet that you can still buy new, so you while many other parts you're stuck with sourcing good-condition-used, you can still get the original Sanwa or Semitsu buttons and sticks that these cabs were made with. it's the only place where you can buy new and still keep it "all original".
Finally look over all of the harnesses and connectors, look for any hacked wires or modifications and clean up or replace anything that's not original. I would recommend using original harnesses over replacement ones and only go with replacements if the originals are completely beyond repair.
5. Cosmetics
Once you've got everything fully function then turn your eye to cosmetics. Consider painting, dying, or whitening any badly yellowed stuff, try some paint-less dent repair. Epoxy cracked plastics, etc. I don't recommend repainting for the sake of repainting.
Personally I like a "players cab" that's one that has a great monitor, great sounds and great controls, it's clean but it looks used and shows some of it's history...
Once you've got the major cosmetics out of the way (dents, cracks, yellowing) then pickup some move strips and marquee art for your favorite or go-to game to finish off the look.
I think cosmetic work beyond that can get very expensive very fast and if it's not done well it can actually make a cab worse. I've seen horrible paint jobs (even by auto body shops) or repro art that has wrong colors, poor materials or incorrect sizing... Personally I'll take original art that's a little scuffed and faded over a bad repro any day of the week.
