Thats why i asked for you to disable the injection and test on either. If it runs on either then your problem is just fuel mixture. If it doesn't then you may have an ecm problem that could be fuel or timming. Try the either trick.
Disable the fuel injectors put a dash of 30wt oil in the cylinders take off the intake tube and give it a shot of either while cranking? Might take the exhaust manifold loose at the head?
1. Change hst and hydraulic oil to sudt? Oem filters? Then let us know if it's better? If you could cut the filters open and unfold the media so we can see how much metal is in the filter would help
Yes the hst can cause your engine to overheat. There's an oil cooler at the filter. If your oil is excessive hot it can overcome the engine cooling. Get a laser heat gun and check the different locations for heat
Zetor made in Czech Republic. Belarus are made in minsk belarus but they sold a bigger 4wd made in Ukraine and another even bigger from Russia. All still available and parts.
Check the oil level in the gearbox? Has the gearbox oil been changed? Check the driveshaft to the front diff and check the oil level in both diff and the outboard hubs.
It's really not a throttle dampner it is for the hst to help prevent the sudden stop if your foot slips off the peddle. My advice is either leave it or remove and throw away and get a coast valve
The tank under the seat is the common reservoir for the hst the power steering and the bed lift. The gear case is it's own reservoir and has no pump or filters. The gear case is splash lube.