Sad, but true.
Since this is a case of oil starvation trashing the engine, the head either needs to be completely rebuilt, including new camshaft(s) and bearings, or you need to replace the long block to get a new head. If you don’t, you will very likely spin a cam bearing and trash your new engine once again, and odds are your extended warranty won’t cover it a second time, and the short block warranty won’t cover it either. Also, from now on it would be best to check your oil on a regular basis. Odds are this situation may have been avoidable. It would be unusual for an engine to consume that much oil in a very short amount of time.
Does the engine have cam bearings?
Has the OP mentioned whether the head would be re-installed ‘as is’ or with normal prep work?
the same mountainbike,
My point is that the usually small cost of checking the heads in the short block option does not really increase the labor or cost of the long block install as the labor is roughly the same. The difference was this guy would not get squat for the extra long block but the engine labor short/long is the same whether he alone sent the heads out for work, then his cost may be much lower than the long block non covered cost.
Euryale, in this particular case I’d be very uncomfortable with just swapping off the heads. But I do see your point.