Hi Alan,
I would find and fit the correct sprocket
I dont know where the 15 tooth came from? is it the correct pitch for the chain??
If so it may be from a C11???? Bigger 3/8 pitch maybe from a Norton or AJ/Match)
E3L's to the best of my knowledge only come in 2 different types , magdyno and taper shaft
(as in one will fit and one wont
)
The belt drive conversions available speed up the dynamo 10 or 15%
Some people prefer to keep the dynamo standard (6v) and use a12v reg
Others prefer the converted 12v dyno
HTH
John O R