These engines are pretty easy to take apart, clean and inspect and even easier to put together again with fresh gaskets and new parts if anything is worn beyond spec.
If you want to keep driving before a complete rebuilt, take off the head and clean everything and go with a fresh head gasket.
The valves will tell you a lot about the history and use.
Do a compression test before and after.
Motor flush is too harsh for a clean engine and can clog the oil pump screen of a dirty engine, if bigger chunks don't get fully dissolved and break free.
I have read too often about folks seizing an old engine going the easy route with just a quick motor flush.
Lots of elbow grease, scrapers, metal bristle brushes, detergents and solvents always do the best cleaning job...
It's a bonding experience with your old iron and you will not regret it.
My old 235 was never really dirty inside when I got her 13 years ago. And after over 30 years on the road since last rebuilt, she is still as clean as new.
I rebuilt the head a few years ago and compression and power is still in the 90%.
Typing this still blows my mind and gives me goose bumps.
Stovebolts are as bulletproof as they come.
Keep 'em clean and running ...!
