There are two overhaul limits used with these engines.
New limits
Service limits.
If you pay more, you receive an engine overhauled to new limits.
Items routinely replaced.
All gaskets, internal hardware (including counterweight bushings), case thru-bolts, etc.
The crankshaft is inspected and reground, if nesessary.
Same for the camshaft.
The engine case is sent out for inspection and line boring, if needed.
Cylinders can be overhauled if desired, and this would normally include new pistons, rings, valves, valve springs/rotator cuffs.
The starter adapter is overhauled.
The magnetos are overhauled.
The propellor governor is overhauled.
The fuel injection system is overhauled.
The engine is then assembled and test run.
Note that accessories are generally not included, such as alternator, vacuum pump, turbocharger, however these can be overhauled at extra cost.
Cost, an interesting subject.
Take a typical engine, say a TCM IO520.
Field overhaul to service limits, $13k
Field overhaul to new limits, $16k
Rebuild by the engine manufacturer, $19k
Note that with the rebuilt engine from the manufacturer, a new engine log is issued, indicating ZERO time.