I dont know about that, because people were making their engines to same spec as sst60 before oval port engine was made ?
oval port did not come until 1993 in most parts of the world.

the oval port head for the 60hp and 70hp is the same part number,
the 50hp 3 cyl oval port is not the same number and is lower compression, I dont know the thickness of the 50hp head or depth of pocket...


on the thin cylinder head why not fill the bottom of the pocket to get correct depth ? then plug hole can then be re-taped and tiny bit machined from outside to
give correct depth for the spark plug.
this would save shaving the head too much by just adding a small amount of alloy in the pocket ?
maybe this is a bad Idea ? but if it can be done then it could be a simple solution to use any of the cyl heads .
for example have 2mm material put in the pock then just tiny shave and its perfect ?

all depends how easy to get thicker head ?

its just an idea that maybe could work I dont know ?
.