The M44 is a pretty robust engine Barry, it was developed from the M42 and both have been successful race engines, I think the problem comes with age, neglect by previous owners and general wear and tear.
If the coolant system is looked after correctly, there should be no ill effects such as over heating, and it is over heating that causes the head to warp, crack etc, I would say you have been unlucky.
It is important with alloy heads that the coolant has an inhibitor to prevent corrosion products forming, it's very much like a heart attack caused by furring of the arteries, in an alloy head the galleries become furred up, and then it's choking time.
The benefits of the M44 is that it has a forged crank, timing chain rather than a belt, a tubular exhaust and variable intake which makes it a very robust and nicely tuned DOHC, one of the best in its time period, similar to the Ford BD series, it's this development that allows it to take a supercharger with out internal modification and not break.
I know it's frustrating,but once sorted, I am sure you will enjoy it.
Where's the picture of that Z3?
