The engine is 40 years old. You are lucky the rings have any tension left in them at all. It is not so much a matter of 1500 hours, or even your 50,000 miles on the odo, as much as the repetitive heat/cool cycles over time. They may not really be "worn out" (end gap), but the tension against the cylinder wall is likely a fraction of what it was new. Come on guys, be reasonable, how long do you expect a medium duty engine to run/last without rebuild? And as others have said, especially with the lower quality of materials of that time, vs todays. You are also running non-approved fuel (WMO), which is totally knarly, but then expect the unexpected. Likely a non-issue here though.
In lieu of that, try to upgrade the materials if possible. It could even surprisingly be less cost than NOS. Start by calling piston/ring/bearing makers, telling them what you have, and ask if they have updated materials which would fit your engine. Things in stock at military surplus distributors are likely to be NOS, which would get you back to near new, but with the same decades old technology.
As comparison, the Kubota engine in my diesel light tower has a 22:1 compression ratio, and the Kubota manual states cranking compression pressure testing should generate 412psi-469psi. Cranking (dynamic) compression is greatly affecting by valve timing events (cam profile), so it is not directly apples-to-apples, but just a rough reference.