Nail head hit. If the manufacturer says 250 watts but it's 750 then you are ok.@Danidl : Torque isn't that relevant here, "if I was king" I'd say a 250w motor running on 36V must have a 7.5A fuse.
If you are hellbound on limiting it for how much power they actually produce rather than what they consume then do so, they manage to do so for both mopeds and cars don't they?
In this case the motor can produce 3kW in real life, but because the company who makes them markets them as "250W" its street legal. just seems silly to have any restrictions at all if you can get around them by just straight up lying...
But this tread has gone horribly off topic now....
If you buy a 500 watt and relabel it, you are committing fraud and driving an un licensed,taxed or insured motor vehicle. In an accident you would be liable for all the costs and any damages on top of any criminal offences.