No I'm fairly certain it's not the timing, i have had it checked.

Is it normally better to have the head overhauled, or to buy a good second hand one?

I'm no genuis when it comes to bikes or mechanical things so it could probably be something else all together. How would I know if it is the piston rigns? Would i Have to open the complete engine up?

N

How do you know it has no compression, I assume you have had a compression tester on each of the cylinders?
That at least will tell you either which valves are not sealing properly or which cylinder is fucked.

If you have lost compression to all 4 cylinders then this is a serious problem and I would recommend getting a mechanic to look at it or just buy a new engine as this would be cheaper. I wouldn't have thought it would be the piston rings as you would have had warnings, like blue smoke etc, it could be a piston or the bloke has cracked, but I would have said it is most likely valves.

With the cost of secondhand engines what ever it you may be better off just getting another engine.