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Dave's suggestion seems good, but since you've already removed the plug, I think I'd go with JB Weld or equivalent, something that will be gap filling.
I'd stick with aluminum for the plug, to avoid a different rate of thermal expansion, if nothing else. Combined with the press fit, I think JBW will hold just fine, probably better than Loctite would. But it would be best to roughen up both the hole and the plug a bit, you actually want some gaps for the epoxy to fill, plus that gives some "tooth" to improve the holding power. Another option might be to tap the hole and put in a set screw, combined with sealant or epoxy.
Glad you got that sorted out, Mike. I've found the Upshur plans to be generally fine, everything is there and seems to be accurate, but the presentation of information for the cylinder head, Sheet 12, is confusing. I'd seen the detail on the next page, Sheet 12A, showing the plugged through hole, but assumed it related to the optional top mounted spark plug, which is the main subject of that page, and really just ignored it. On the main drawing, in the top view, that hole is shown as a dashed outline, which I read as a the hidden outline of a blind hole. Apparently, Hamilton meant it to show the outline of the plug! I now realize that is really an assembly view, and the drawing to its left, which I took to be the bottom view (but wondered why it was not a mirror image). A section view A-A through the spark plug hole is indicated, but is not found unless you look to the next page. A section view through the intake valve is shown, but not called out. As I say, confusing. At least the problem you had helped me finally sort it out in my own head, so Thank You for that, Mike!I don't understand why he didn't just show a hole drilled up from the cylinder side to meet the spark plug hole, which is what I intend to do and you've realized you could have done.
I can give you another hint to avoid a possible problem. The rocker arm pivot mounting hole shows to be drilled thru and the round .1875 section is 1.00" long. I did it as per his drawings and most probably since there's more length there, it did seal. However, if I do it again, I'd drill down to perhaps .40" and shorten the rocker post accordingly. That way there's no penetration into the cylinder area to give problems.