Looks like a C20XE, similar thing happend to me (water pump locked up, pistons meet the valves).
I replaced the pistons (kept the rods).
And all 16 valves. The bores were re-hones (so engine out time).
Other than that, that was the only 'mechincal' things I replaced.
I was only going just over 1500 revs when mine locked up, so the valve guides were not damaged either (had them checked out).
Once those were repalced I got the normal serviclbe parts repalced, ie
1) cambelt, alt belt, water pump, rollers, tensioner
2) head, inlet, exhaust gasket, head bolts, piston rings
3) valve oil seals, re-lapped new valves in
I decided to do a bit more and recon some of the parts while it was in bits.
Yeah it is/was an 20XE, had just gone from 3rd to 4th so the revs were, well, not 1500
It was actually the alternator belt that snapped and went between the cambelt cover thru that little rubber sealy thing n started to rub on the cambelt causing it to fray and depart from the pulleys