I would suggest a couple of options:
1) If you have access to the backbox and/or lifting the playfield, i would scrounge around looking for the key. I have found many...MANY keys this way.
2) You can sometimes use a screw driver or butter knife to open it. If it is possible, open and look for the key hanging inside the coin door or on the inside of the cab.
3) If you have other keys laying around, you can test them. Some operators and collectors use "common keys" which open up a ton of different games. It's worth a shot. You can also pick the lock if you have the tools or skills.
4) Drill it. 1/4 to 1/2 bit are typically used to drill straight down the center and gain access. Replacement locks with keys run about 4-5 bucks. Look for double bitted 7/8 barrel/cam lock with keys. Home depot, lowes, walmart, Amazon, and eBay are great places to find them.
Here is a good example: