Anyone who owns a bicycle can use the Asus Oxiis E250G1 Intelligent Bike Booster to transform it into an e-bike in just a few simple steps. Equipped with a cadence sensor, it is designed to respond just as quickly as a conventional e-bike, and can be charged via a USB-C charger.
These are just generally not a great option compared to a hub motor retrofit IMO. Honestly I’m not even sure what the use case is unless you have some very obscure bottom bracket which can’t accept a hub motor.