MRC stands for Model Rectifier Corp----evidently they used to import Enya engines, so what I have may be similar to an Enya. Fortunately, I have the original prop nut---it was a better way to use an electric starter that I was after. I'm going to try a plastic spinner from DuBro, hopefully that will work. Turning the cup around didn't help much, nor did anything else I've tried so far.
The number of threads is exactly 28/inch----the diameter may be 7/32, or 5.5mm, or #12----without a known test sample, I can't be sure. Those sizes with a 28TPI are not to be found in hardware stores, at least in the four I've tried.
I sure appreciate all the replies to my question!