Bob, this is the same area on the mini-tractor:
It looks kind of similar to me, but you are missing everything from that picture except the engine and the innermost part of the pulley/clutch system (Item 7). I'm guessing that the fixed, inner pulley (Item 7) drives the back wheels, and all the missing bits drive the cutter. Maybe the removal of all of that, and the substitution of a lump of wood to tighten a belt, constitute the previous tenant's improvements?