The Onan engines don't have compression releases.
50 PSI compression sounds very low for an engine that starts and runs well, and develops power.
I'd question your gauge accuracy and technique (from this distance).
Rings? Valves? Use the old standard test of squirting a little oil in the cylinders and retesting. No rise in pressure means valves; good rise means poor ring sealing.
Another check is to turn the crankshaft by hand and feel the compression. Two turns for both cylinders, and you should feel compression on each revolution. Listen in the carburetor with the throttle open and in the exhaust for wheezes (valves). No compression when turning by hand usually means valves.
Poor rings gives blowby, with oil blown up through the breather into the air filter pan.
Also, blue smoke in the exhaust and oil deposits on the plugs.
These are some secondary tests to confirm accuracy of compression tests. You can also use a leakdown tester.
Hank
