Currently costs to modify old cars entire drivetrains would be (a) too expensive (b) not viable due to the size (quantity required) and placement of batteries.Shouldn’t the car manufacturers be working on a retro fit engine and battery pack that prevents adding billions of tonnes of waste to the planets scrap heaps ?
A theoretically good idea, but one that would need a jump in battery technology and overall manufacturing costs. Fundamental drivetrain is different - HOWEVER. If someone super smart could re manufacter a 1.4 turbodiesel engine to slot in an electric version, put the batteries in the boot, then possible - but would affect handling, giving other problems.
Battery's on the new cars tend to sit underfloor spread out, to lower centre of gravity. Stick a couple of hundred of kilos in the bootm in your old hatchback and it will nee the rear end re-engineering too!
If you are looking to save the planet, how about synthesizing a compound which breaks down plastics organically within salt water - thus fixing the oceans plastic problem. That I suspect is a solution already being looked at?