I don't understand the "the car is toast if a valve hits a piston" comments. Are valves and pistons not available? Because changing them is easy as well.
I actually had an exhaust valve smack the piston on the Sporty I use to have after replacing cams and the timing mark on one was off a tooth. It hit dead on and didn't even bend the push rod. It was no worse for wear. Sanded the little dent in the top of the piston to avoid a hot spot, checked the valve for leakage, and threw it back together.

 

Gunner, these cars do not have motorcycle engines in them. To replace a bent valve in these Jaguar engines is not a walk in the park like a motorcycle engine is. How many set of heads have you pulled off of these Jaguar AJ engines while still in the car?

 

I was lucky it cost me 200$ labour and 300$ for parts . We did it in 3 days my friend the mechanic remember the timing cover and I did the rest pretty easy

 

Seriously? How hard is it to pull a head? C'mon.
What makes it so much different than the head off any other car of which I've pulled many in my day? Is it really that bad that one would just write an engine off?