In my experiance dealing with 50 non running XJSV12’s unless you get a bad leak down reading indicating a valve problem you won’t find enough wear to justify rebuilding.
Due to its extremely short stroke ( piston speed is really slow even at 6500 RPM.) ring wear is almost never a problem even with 200,000 miles on them.
The guides typically go well over 150,000 miles before wear exceeds OEM specs. The crankshaft bearings are massively oversized and sitting behind that automatic seem to last foreve even on race cars.
The only weakness is if someone turns the engine backwards the cam chain adjuster will pop out requiring pulling the timing cover off and replacing it.
I understand curiosity and just wanting to get in there but it really is well enough built that typically it’s not needed.
Yes I’ve rebuilt a few for racing. But now I just change camshafts add a pair of turbo’s and clean the exterior up.