I purchased a used airplane at a very good price. I figured I could sell the OS 91 that powered it at a good price and replace it with my DLE 30.
However the OS does not seem to be in good shape although it looks good. Apparently it has been sitting awhile.It is hard to turn over and the compression seems to be down. The bearings are probably full of congealed castor or other oil.
Do you engine experts think it would be worthwhile to install a new ring and free up the bearings then sell it?

I wouldn't sell it with the old bearings unless you let then know. Bearings a cheap at Boca Bearings. I would strip it down and clean everything and put it back together and see how it feels ,may not need a ring.

load it up with automatic transmission oil and let it sit, then turn it over and see if it loosens up you might be able to tell if the bearings are bad or not.