Been there a few times - and sometimes I find the backdoor key inside the body. I know that sometimes ops removed them from the front door hook and drilled them into the sidewall of the cabinet. Also, look in the corners - where the dirt and dust have accumulated. Another place is the coin reject chute.

Don't give up - Lift the playfield up and spend 5 minutes - even check under the baseboard (the plank where all the steppers ect reside).

Last resort - DRILL the old lock. If the backglass is still in the head you may risk breaking it with the impact of a hammer. Use a smaller bit and move up to larger bits.