If you fit a bike with a motor kit more powerful then a 250w continuous rating or wish to legally ride it and above 25km/h then one has to get the bike essentially MOT'd. One has to present the bike to a MSVA testing centre to be checked and then a certificate issued.
Your issue will be complying with the rules on how the bike should conform, basically it has to be up to moped standards so indicators, proper lights even a brake light, a kick stand etc etc. The effort to do so is not worth the aggro or cost, as well as this one has to then insure it for third party and theft as well as wearing a motorcycle helmet full face or open face.
A testing station will guide you in to what needs to be done for any said bike to pass the inspection, end of the day do you really want such a massive headache.
For speed pedelecs they should have a certificate of conformity which will make the process straight forward but for self builds, a mine field and I think we only have had one person on the forum go through the process some 6 or 7 years ago, the bike looked anything but a normal day use bike.
Tbh just fit and buy a 250w legal rated hub and ride it and have the benefit of a pedalec, a lot of 250w motors can temporary output 500 - 900watts of power so why the need to use a over rated motor ?