I managed to get the Honda MT50 mentioned road legal in the UK.
The thread is here
if you want to check up on the details.
The 50cc bikes of the "slowped" era are a lot lighter than the 125cc stuff, which is the main reason I chose this one.
The main advice I can give is to decide if you want a moped class or a motorcycle class - the moped class is limited to 4KW and 30mph.
Changing the class of a moped to a motorcycle with the DVLA is probably just as much hassle as getting them to swap the power from petrol to electric, so best if you can get the right donor to start with.
I agonised long and hard over 50cc or 125cc... in the end I went with light weight.