Legislation for a mobility scooter is not the same as an e-bike and is 4mph for pavement and 8mph for road use, so no matter whether you make it go 16mph or 40mph is irrelevant, if your using it on the road.
Depending on you scooter make and model? you can change the sprokets by putting a larger one on the front wheel and a smaller on the rear and it will go x3, x5… quicker without messing with any electronics etc.
The other option is a 3 or 4 wheeler e bike, if your wanting to stay legal.