1, go get yourself an owners, service and parts manual.
2, common sump rear, tranny, and hyds.
3, oil: if in cold areas.. get a utf oil that meets the M2C134 spec. If in a less cold area.. a 90wt gl1/2 gear oil.. or a 80w90 gl4/5 MT1 rated oil that states it is yellow metal safe will do.
note: utf oil may leak if your seals are rotten.
axle seals? go sure seals?
trans input shaft seal? 4$.. needs a split to install.
pto seal? depends on how easy depending on the age of the 9n.. in any case.
sn of machine sets age. you can look for clues to see if the engine sn matches the rest of the tractor looking at casting date codes and compairing. if all are within a few months.. .. etc..