you can get some timber strips from hardware or 10mm half round or square and glue this to the cabinet bottom . either remove the old strip or reglue existing trim/ply if okay. i did remake the rear of a gottlieb cab using 20mm ply but its a bit of a job because you need to rebate it along the corners. or put a post out for a cab im sure someone near you would have that era gottlieb cabinet.

If you want it to look mostly original you could do what I did to a Williams cabinet.
I cleaned up the bottom of the sides so that the groove was completely removed and the remaining plywood side was 8mm thick.
I then cut strips of 12mm plywood and glued and clamped them in place.
Cut the strips a bit wider than the step from the baseboard to the edge of the cabinet sides so they protrude above the bottom of the sides and when the glue is dry, trim the excess off with a router.
Then end result is from underneath it looks just like the machine has 20mm plywood sides just like it did originally.