I have a za that was my fathers that he rebuild and has had a knock since. He told me it was caused by cam shaft end play and it wouldn’t hurt a thing to run it.
Its still running 50 years later. I use it several times a year and it will always start and runs just fine. I even plowed with it last year.

don’t be in a hurry to fix a knock if your not going to be pulling it hard. Cost will be more than the tractor is worth.

take care

pointrow