As you have already said Dave, one answer is to make a compression plate of suitable thickness to go under the cylinder, possibly with longer base studs to suit.
What has baffled me is why someone would shorten a cylinder like this but I now have a theory. Perhaps someone wanted to use short stroke flywheels (= 350 flywheels) and a VA piston. The VA pistons are vastly more common than actual VA bikes and are 81.8mm bore but have a much bigger distance from gudgeon pin to piston crown. So if a +0.030 VA piston is of any use to you, let me know. I only bought it for the rings and gudgeon pin as spares.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), 54 4G Mk IV (project), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser.