Your model number should be 91-12K802 and that would make it a 1979 C-121.Nice tractor. you can get a parts breakdown from Toro here:
https://lookup3.toro.com/partdex/index.cfm?xCaller=Toro
You can order manuals plus download several here: https://lookup3.toro.com/request/request.cfm
Put "none" for the serial number.
Here's your engine service manual: http://www.kohlerengines.com/onlinecatalog/pdf/tp_2379.pdf
That should get you started. Good luck.
https://lookup3.toro.com/partdex/index.cfm?xCaller=Toro
You can order manuals plus download several here: https://lookup3.toro.com/request/request.cfm
Put "none" for the serial number.
Here's your engine service manual: http://www.kohlerengines.com/onlinecatalog/pdf/tp_2379.pdf
That should get you started. Good luck.