Looks like the crank's been replaced, along with the left-side front shifter setup. Apparently someone had a need to swap those out, or scrounged that particular bike for parts they needed for another build.

Not sure if you've tested the shifters yet, but they're likely to be jammed up. More damning is that the left (replacement) side has Shimano's original Rapidfire design. I'd dump them both in favor of something a lot more simple/efficient - Suntour XC Pro, Shimano Deore II or Deore XT thumbshifters are the ticket. You can then split out the brake levers in favor of some Dia-Compe SS5 or SS7 types. They work really well with Shimano's low-profile cantilever brakes.

eBay is about the only consistent source for a lot of vintage era replacement stuff, so I'd start there.

Vuelta makes pretty good pinned and ramped chainrings, assuming you can't scrounge for (round) replacement Shimano SG types.