Friction drive e-bike conversions were popular years ago. They generally: * Wear tires surprisingly quickly * Absolutely suck in any form of weather or terrain condition (dirt, rain, etc.) * Have ~20% less efficiency than any other drive form. But, they are easy, and they do work.
This bike booster will probably come in at less than half the cost but the Bimotal kit is another option that doesn't have the tire wear issue, offers better performance, and the form factor is much nicer. https://bimotal.com/products/elevate
A proper e-bike needs to be designed as an e-bike from the beginning.