i have been looking into this myself in the uk.
it turns out it does not matter what cc engine you have if its motorized by a fuel combustion engine you have to have it registed taxed and insured.
if its electric you dont need anything.
the police here dont really bother you if your not doing anything stupid or causing anoyance.
This will surely be of interest to both of you. Note the aux muffer on his bike, it might ding the output by 10%. Note the requiored dBA rating at 75% throttle, a Chinese engine may not make this mark.
I get another answer from seller from Poland. He said to not bother about it. That silly but he said there is no regulations (in Europe) which could affect ride on it. This is not a moped because it wasn't there when you bought bike. This is KIT and only helping device. Germans buy them from him to bikes as well (they doesn't mater about limits because engines are unsigned). I'm on 99% trust him. That 1% is KILL SWITCH .