The only difference between a 1953 KHA and a 1954-57 KH is that the KHA has an alloy cylinder with cast iron liners wheras the KH has a cast iron cylinder. The two types of cylinder are interchangeable. The alloy cylinder has a tendency to crack the alloy. The central bolt between the cylinders will also strip the threads very readily, as per the threads in alloy Square Four heads. The alloy cylinders tend to be noisier than the iron ones.
I happen to have an alloy cylinder on my 1954 (but early 1955 model year) KH because it was the only one available when I had problems with the engine shortly after the rebuild. The alloy had cracked but had been welded back together. So far it has done about 15,000 miles without trouble. I have another (much better) alloy cylinder to go on the ex ISDT KHA when I start that restoration once I finish the Healey (which is coming on well).
If you paint a 1954-57 iron cylinder with aluminium paint and avoid oil leaks to stain it, you may have the best of all worlds.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.