The original one is worn and hs a few cracks. It would be wiser to get a better head and overhaul it.
I don´t mind getting a very different head, but which?
C heads are very expensive. B heads are better than the A heads used in even the 150S (more torque at lower revs). Later XJ heads with bigger valves, need a bit of work as water passages are different at the rear.
Itś a 140 DHC with no racing ambitions.
If originality doesn’t matter, the best head by far is to get a readily available B type head of any Mark 2, S type, etc, and preferably one that hasn’t had previous work, especially anything off gasket face, nor any signs of overheating.
Similar power and torque to a C type head, thus better than an A type, and none of the major problem s you will have to resolve with an XJ head.
As roger has stated but remember with any straight port head its not just waterways but inlet manifold to consider.