You'll likely never find the parts to reconvert, game is too rare. Everyone I have known who did or tried to deconvert a Food Fight had a bear of a time finding the parts and spent as much or more than what one is worth. The parts just aren't really out there. Checking ebay shows no Food Fight parts in either current or completed auctions.
Checking RGVAC shows requests for basic parts (such as joysticks and bezels) repeated over and over again by the same posters for months and YEARS with no responses. Hasn't been a joystick listed for 4 years, I can't find a bezel ever being listed at all.
The boards can be had though. Its the joystick, control panel and bezel that are the problem.
I was going to suggest just cleaning it up and also getting a Mortal Kombat board for it (Mortal Kombat and High Impact boards will swap directly WITHOUT having to readjust the monitor, swap anything else for a High Impact board and you will have to readjust the monitor), but then I realized it wasn't yours, so it is a moot suggestion.