Take one old motorcycle with a lot of miles on it. Preferably something that leaks a bit of oil, has trouble running when wet, clutch is slipping, tank contains more rust than gasoline and so on. In other words something like this Kawasaki Z500 -81 - only three years younger than your's truly:
a conversion kit or separate electrical parts from your favorite online EV supplier. This is the part where I am currently at. I have pretty much decided on a 72 volt system with a brush type DC motor. The motor will probably be either ME0709 or ME1003 mainly depending on how much money I'm willing to spend. The batteries will initially be old UPS AGM type, but if all goes well and the bike passes the Finnish equivalent of MOT test after conversion I should be switching to LiFePO4 as soon as possible. This will also require a proper charger. I am planning to forego BMS completely and instead relay on the bottom balancing method as described on EVTV on numerous occasions.
ps. Let me know if you'd like to buy the engine. It's actually in a pretty good shape.