I have found that the space between the mag pinion and the crankcase housing is too narrow to fit an oilseal,
Magneto Oil Seal - 20 X 38 X 5
The cavity is 5.7 mm deep so Rich's suggested 5 mm-thick seal wouldn't even protrude from it. The cavity is 1.50" diameter (38.10 mm) so I assume that extra 0.004" is within the ability of the oil seal to accommodate the extra crush.
The seal actually measures 38.10 mm diameter, 4.75 mm thick.
I seem to remember putting something on the outer edge of the seal just be sure, likely one of my usuals of either 3 Bond product or Loctite 518. It didn’t fit loose, took some pressure to instal.