Yikes! That's some nasty butchery. You could hunt for a cooling fan on the internet...try Parts Express, Mouser, Digi-Key. Or clean up the hole, cut and glue a plug made from 3/4" plywood and recover the head with replacement vinyl available from Angela Instruments or the vendor on eBay that also sells it. Or you could have any number of cabinet makers build one to match your old cabinet.....send them dimensions and photos and some Sunn tolex. Mojo, Rodgers, Sourmash and many others could do a fine job. Then move the hardware and grill panel over to the new cab.
Good Luck!