The alloy heads are more prone to distortion and damage caused through misuse than the iron type so must be examined carefully at the point of purchase...
Check the spark plug thread is OK..Check that there is not excessive wear where the heads of the headbolts sit when tightened...(They should have a heavier than standard washer fitted under the bolts to prevent this)...Check that the thread for the timing plug hole is OK and that the bolt isn't stuck in the head (corrosion). Check that the gasket face is flat..The alloy heads are more prone to distortion if unevenly tightened...Use a solid copper head gasket and torque down evenly to prevent this..They can be skimmed lightly to remove distortion as can the head bolt surfaces..You will also need a different set of head bolts.... The alloy head uses ten that are the length of the longer ones for the iron head...The spark plug is a long reach type..Grade Champion N5C or equivalent....
Adantages of the alloy heads are that the engine will dispose of excess heat much more efficiently and they don't rust..I've run one for years on my WD bike...I painted it black to disguise it and it has rarely been noticed by anyone.....Ian
I've owned three M21s. All had iron heads. At least two should have been alloy..... Back then we didn't have torque wrenches much and we didn't have solid copper gaskets.
Slightly off topic, but you see so many badly damaged iron heads with multiple fins missing. These are bloody heavy, tough lumps of hard metal. It makes one wonder just how cruel & downright brutally careless were some of the past owners of these machines?