I've looked into this recently and think buying a conversion kit to fit on a standard bike seems to be the way to go.
You can buy some really nice purpose made ebikes but they tend to be expensive and underpowered.
There are two basic types of motor kit available - hub motors and mid drive motors, mid motors look to be the superior choice. You take off the front derailleur (you will lose those gears) and remove the bottom bracket spindle, the mid drive motor then slots in. You can still use the gears of the rear derailleur.
Bafang pretty much dominates this market and you could get a 1000w or 1500w motor that's good for 30 or 40mph respectively.
https://bafangusadirect.com/products/complete-bafang-1000w-bbshd-mid-drive-ebike-motor-kit-battery
Just the kit no battery - https://www.amazon.com/BAFANG-Electric-Chainring-Conversion-Mountain/dp/B0995G3X3Y
You want to make sure you don't cheap out on the battery because the dodgy ones can catch fire.
Good luck and let us know if you do get one.
Oh and something cool - DARPA developed an ebike for paratroopers called the Tidal that was a neat concept https://newatlas.com/the-highly-desirable-1000-watt-limited-edition-tidalforce-m-750-x20-electric-fold-up-bike/11674/
1000w exceeds limit in the state
Gay. I bought a 1000w conversion rear wheel with a 48v battery. It goes 28-30mph easy. I have broken 2 frames so far because I mounted the battery off the seat post. If you have to ride in the street 30mph is not too bad.
They do a 750w option.
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