Mike is referring to the needle roller bearing inside the rear end of the input shaft, this would not make the shaft seal leak as the purpose of this bearing is to locate the top shaft in the gearbox, not locate the input shaft. ...... you will be able to check it if you have to replace the input bearing, as you will have the whole input shaft out (most likely).
I have not done one on a
Zetor , (Lots of Mini's and Toyota Celica boxes) every box comes apart differently so you will need to refer to the
manual ................. cross fingers it is just the seal, but if it is jumping out of gear it is likely the bearing as well.......
one other thing to check (if you have not already) check the bolts are tight between the engine and the gearbox, if this is not aligned correctly (should have dowels.... i assume) you will get the same problem....... remember the front bearing on the input shaft is the one in the back of the crankshaft! so the gbox and the crank must line up ..................
This was a common problem on old english Fords (Prefect, Zepher, Anglia, Cortina etc) .... first symptom was jumping out of top gear.
A mate of mine drove a 2L MkII Escort around so long with the bell housing bolts loose, the startermotor fell off! when i went to fix it there was only 1 bolt left in the bellhousing and yu could get 2 fingers in the gap!