The problem, of course, is mostly kids and teens riding
A powered vehicle can do damage. The solution is to only permit responsible people access to powered machines and then hold them accountable for any damage they do. We already do this.
It's called a Driver's License. We already understand this, and we understand why. The only change needed is to require a Driver's License for any user of a powered vehicle (apart from the already-defined and already-regulated use of disability scooters). And then enforce the existing framework for licensed drivers.
We permit people to modify their petrol vehicles, and so we already have the laws and frameworks to deal with DIY electric vehicles. There is no objection there if you notice this and if we stop attempting to twist ourselves into permitting children (who by legal definition are not responsible) access to powered vehicles, then we have no problem with "modifications" or "DIY".
There is no sensible argument to be made for contorting ourselves into odd new schemes for approaching this. Powered vehicle requires responsible operator, a Driver's License specifically is about granting permission to a responsible operator, and we need only correct our laxities in current enforcement to make this work. It's already exactly what we want and need.
I don't see why supposedly intelligent adults continue to insist that children should be given powered vehicles. Children can't be relied on to behave properly so don't put them in situations where that is life and death, either for them or for others. Children also need to use their bodies to grow properly. Why is this even a question? For the same reasons we don't permit children to drive cars, we don't give them other powered vehicles until they are not children.
The only change needed then is to improve enforcement of adult drivers of any sort. If you want a class of DIY bikes permitted, get off your ass and get involved in putting through the laws and you'll get what you want that way, instead of twisting yourself around in weird ways to inexplicably insist that children should be permitted access to powered vechicles and insisting that everything should be changed so that they can and you should nevertheless have no restrictions on what you can do.
This is stupid..
Edit:
People don't like being told their choices are stupid, whether they are or are not. In this case, I wrote about the various sideways efforts to permit children actions which they are not ready to bear the consequences of, and the weird sideways efforts to somehow blame this on the technology while simultaneously attempting to
also give carte blanc to adults do do as they wish under the same approach. Those efforts, to me, seem obviously contradictory so insisting on them simultaneously rather than addressing responsibility and who can be considered responsible does seem directly stupid to me.
Nothing about requiring a Driver's License in Public spaces prevents, say, permitting your children to use quad bikes to herd the sheep on your private ranch.