You've got me wondering if it's still not permitted to sell Stanadyne pump parts in non USA areas? Roosamaster/Stanadyne - created by Vernon Roosa - back in the late 50s sold rights to CAV to copy the USA built pump. At that time, the agreement was - no Stanadyne pumps or parts used overseas - and no CAV or Bosch rotary pumps allowed in the USA. But, those agreements changed over the years. Deere was one of the first to use the rotary pumps with the 1010s and 2010s in 1960. Hercules, et. al., beat them too it.
A complete seal kit costs around $12 for the pump.
SPACO in Italy sells them.
Deere used two models. The model "C" and variations of the model "D." No parts are available for the C series. It was a mid-range pump used on many 350 and 450 crawlers, 1020, 2020, 2520, 3020, 4020 tractors, etc. Deere sells a conversion kit to adapt those tractors back to a D series pump.
Deere sells tech manual SM-2045 for repairing the pumps. It is also available in reprint from many tractor websites for around $30.