Rebuilt:
https://www.tractorpartsasap.com/cylinder-head-remanufactured-kubota-v1512-209955.html
Used:
https://www.tractorpartsasap.com/cylinder-head-remanufactured-kubota-v1512-209955.html#block-related-heading
New:
JUMP ON IT:
https://www.ebay.com/itm/375305944971
And the head gasket...
The engine serial and model number is stamped on the flat of the block, facing up, right beside the injection pump.
DO NOT go by any castings on the block or the head.
Buy a stock oem Kubota gasket, not aftermarket!
Should be a V1902...
I personally avoid PTFE tape, I use PTFE dope.
There is no standard, as they are both supply, it just matters which way the valve makes it at the time.
Safety (liability mitigation), if they put a short time line on it less chance of issues coming back to bite them.
Most antifreeze's will last long after the recommended time frames.
If you look at the linkage on the governor there will be two slots only the left side is the right slot.
It was a poor design, fouled up quite a few installs, hence the labels.
NOTE: some models only have the one slot.
With fountains like that you have no issue with the injection pump.
You more than likely have not gotten all the air out of the injector lines, it takes a lot longer than most expect.