The right engine on my private aeroplane is a Continental GTSIO520 model.
340 BHP, spur geared propellor drive, fuel injected,turbosupercharged.
It was overhauled in 1990 by Butler Aviation in Tulsa, and after 1100 operating hours, still runs like a fine watch...very smooth and quiet.
I would have to say that it has provided quite good service for its age, I certainly have no complaints.
Overhaul cost?
Depending where it's done, the price would be anywhere from $25,000 to $38,000, including accessories.
The best part about flat six aircraft engines is that if cylinder problems are found, just bolt a new one (or more) on, and continue to operate.
The bottom ends of (especially) TCM Continental engines are very reliable.
How reliable?
PrinAir holds the record, set long ago.
This small airline operated 17 deHavilland Herons years ago, re-engined with TCM IO-520 engines.
FAA allowed time between overhauls?
4000 hours.
Close attention to detail by pilots and ground engineers allowed this long overhaul period.
Treat your piston aircraft engine right, and it will last quite a long time.
Do otherwise...expect large repair bills.