The C-121 was a newer (1978-1979) version of the C-120 (1975-1977), Wheel Horse and Toro often just used an addendums to the older parts and service manuals for newer models. There may be a C-121 Part/IPL manual out there on PartsTree, but it is probably just an updated microfiche file from Toro. You can use the C-120 parts and service manuals for the C-121 and consult the addendums for differences. The C-121 was made 2 years and in 8 speed and Hydrostatic drive models 1978 C-121 8 speed was model 81-12K801 Hydro was model 81-12KS01. The 1979 C-121 8 speed was model 91-12K801 and the Hydro was model 91-12KS01. There are 4 or 5 addendum's for the later model C-121 so for changes made in newer models and others for Belgium made models. I have attached the addendums here and the service manual. The way to read model numbers form 1976 to 1980 Wheel Horse Tractors is the first number is the year, not sure what the second number means, the next two numbers are the horsepower, the letter stands for the engine brand, K= Kohler, B= Briggs, etc. The next number was was 8 for a 8 speed, and 6 for a 6 six speed or S for a Sundstrand Hydro. Model 91-12K801 was a 1979 12 hp kohler powered 8 speed Model 81-12K801 was a 1978 12 hp kohler powered 8 speed and Model 81-12KS01 and 91-12KS01 were 1978 and 1979 hydrostatic units
Tractor 1975 C-100 C-120 C-160 8spd IPL #803983.pdf
Tractor 1979 C-121 8-Speed 91-12K801 TIPL.pdf
Tractor 1979 C-121 8-Speed 91-12K802 Details & Attachments (1).pdf
Tractor 1979 C-121 8-Speed 91-12K802 Details & Attachments.pdf
Tractor 1979 C-121 8-Speed 91-12K802 TIPL.pdf
Wheel_Horse_1978_b-c-d-series_service_manual.pdf