Know idea where the spin on adapter came from. Whitey, the '63, also has a gear reduction starter and has been converter to an alternator (negative ground). Will post a few photo in a day or two. Whitey is outside and has an inch or two on snow covering him up.
I think the spin on adapter from a Sprite/Midget should work as the E1 is a copy of the BMC A motor found I all of your early Sprites, Midgets, Morris Minors, etc.
Again, not sure if the Sprite/Midget spin on adapter works, but here is the part number from Victoria British -
9-1067, cost is a little steep at $124.00. Number is 1-800-255-0088. or go to
www.victoriabritish.com and take a look.
Let me know what other info you might need,
truckie