HI,

If d a timing belt breaks, will it destroy the valves and alloy and camshafts? Or does it destroy just the valves / pistons?

That depends on how fast the motor was turning when the belt broke. Usually it is just the valves that get bent. They may take the guides with them. If you are really unlucky a piston will be too damaged to reuse. I don't know what you would have had to done wrong to bend a cam or wreck the head, but I'm sure it has happened.

Having said that, the cheapest repair might be to replace the entire head.

Greg

Thanks
Not me, just a question when looking to buy 2nd hand head

Depending on the damage and the total cost of the repairs, it may be more economical to buy a complete used engine from a wrecker (if you can be convinced of it's miles and condition). The downside to that is, even if the seller guarantees the engine to be good, if it turns out to be bad it's YOUR labor that has to pull it back out again.