Its a nice idea which probably has a lot of complex implications. It would probably be a huge pain to figure out dimensions and compatible electric motors for every brand of non-electric vehicle, so the production of replacements would become very wide. Typically, the battery of an EV isn't just a brick in the engine room, but it's a whole range of cells along the length of the vehicle. Using the same space as the combustion engine might leave you with a vehicle with terrible range. Also, the safety of a car takes the engine into account. Replacing a combustion engine with an electrical engine would likely require a whole new safety overview for each individual model.

I honestly really hope that your suggestion would work, but I'm not expecting to see this becoming a wide solution before EVs dominate the market anyway.