Evaluating engine lower half on 1950s engine with ~600 hrs and 40 yrs SMOH

Thanks for the quick reply, Larry. Oil temps and consumption are reported to be normal. I was wondering about borescoping as you suggest, but how can you get to the crank, cams, lobes, etc without removing at least one cylinder?

Hi folks.  Looking to purchase an Ercoupe in the 1950s or 1960s range.  Evaluating options thru the listings, FAA data, and info from owners.

One has me stumped. Good looking plane flown regularly. Mid time C-90 engine after overhaul based on hours, but overhaul was 40 years ago. Compressions and other top data look good.  But the problem is I don't know the condition of the lower end. And plane does not have a spin on oil filter and no record of oil analysis, so I don't have A&P reports or data to infer the condition of the lower end other than it was airworthy at the time of each service.

Is there any other way to evaluate a 40 year old engine condition with no oil filter and no record of oil analysis? Doubt the owner will allow removing a cylinder to inspect the lower end parts. As a prior aircraft owner, I adhered to focusing on condition rather than just hours and years and used engine analyzer, oil filter imspection, oil analysis, as well as the typical compressiond and plug condition.

What are the options besides valuing this as a likely runout engine, despite the fact the owner seems to fly it regularly?