I stumbled across a Chinese electric motor with a four speed transmission that will bolt into any Honda 50/70/pushrod 90 or similar clone (or an OHC 90 with the Dr. ATV adapter kit). It appears to basically be a set of standard horizontal engine cases without a hole for the cylinder to fit into and instead has an electric motor where the stator normally goes.
Throttle adapter for ATV, motorcycle, electric motorcycle
$599 to your door, or so the website says. Might be an interesting start to build an electric Cub.