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I drug the 1872 in the garage out of the heat to check it out. Back ground is after about 10 minutes of mowing it quits. Seems hot. I cleaned carb tank even put an electric fuel pump on it new petcock and screen good clean fuel. Same condition. I’m thinking ignition. Plugs look good good spark. ? Did a compression test cold 35ish both sides. Added oil retest 90+ Both sides indicates rings bad valves good? So my question is has anyone had success just doing ring and hone on the m18? Shows 1200 hrs. Not smoking badly or knocking actually rubs good other than dying after warm up. Ring and hone I can do, total rebuild is out of my comfort zone

Use an in line spark tester on one of the plug wires, test when cold and running for a base line. Then once it dies install tester and view spark. It does sound like an ignition issue.
With worn valves/seats mine would run ok... but it wouldn't restart once it was warmed up
I do think rings and valves are possible. I would pull the engine and work on a bench. Very important to set the gap between valve and tappet. I made a jig for my bench grinder.

Do you have a pic of the jig?

In my "magnum 20 rebuild" thread.....august 14th

Corey - What dag said... Sounds like classic magneto failure. Test for spark in some way as soon as it quits.

Remember, the magneto in these engines is a "lost spark" system, there is just one magneto that fires both cylinders on both the compression and exhaust stroke. You will lose both cylinders when the magneto fails. Saturday job, pull the engine, replace magneto, reinstall. Don't even think about doing it with engine in the tractor.