That old of a machine, may have a MTD label on the lower left side of the transmission "box."
There should be a model number. I'd see if parts tree could cross reference it, in the search function. Worth a shot at least.
MTD....2 stage(modern 8.5 HP) not my favorite
Honda HS622 TA-B - acquired at an auction- nice machine for it's size.
Toro S-140- Picked it up on trash day... it runs, sort of.
Yamaha YS 240 TB "Ricky" latest Craigslist find- on the bench now.