Compression is so low on a Hand-Start JD that a compression test should not be necessary...YOU can tell if the cylinders are Leaking down badly..
Maybe the appropriate question should be: How fast of a leak-down would be too much.. If you need to turn it over fast, to feel compression..it is leaking down too fast..!! You should feel little, to no leak-down with a good Fresh engine.. Accumulated carbon, debris and burning from leaking gases will allow valve leaks long before ring wear would be the cause..BUT... If the time of the last re-ringing is totally unknown, put new rings in when the head is off for valve-grinding...it is pretty easy and may not be as good as a re-bore, but with GOOD Honing, will improve the compression for many years..certainly until it needs another Valve-Job... A "GOOD" Valve Job is not necessarily what you will get at JD.. I had to re-do a JD 50 head that JD had done... They had simply dropped New Intake valves on to UN-re-conditioned seats, and those seats were the absolute worst-looking rusted, pitted seats you will see (and they called it a "Valve Job").. Take it to someone who Will do a good job..!!