I think it is hard, as the obvious place to but the batteries in the car is as low as possible and between the axles. In a small ICE vehicle - depending on your definition of small, there's not that much space usually between the axles. Then with the batteries between the axles, there is not much space just underneath the seats. Look at the e-Golf battery pack as an example https://i.ytimg.com/vi/DFCKKzUCRRs/maxresdefault.jpg

You could in theory put batteries in the Engine compartment and Boot, but i think beyond DIYers this is unpopular because of the handling effects, loss of luggage space and, I assume, safety in a crash.

The BMW i3 is I think the best example of what type of small car could be converted to an EV. It would have to be a small vehicle with large wheelbase, with small wheels pushed to the corners to make maximum space for battery modules. It would convert a small hatchback into a crossover once the height was increased so the battery pack wasn't too low the ground or taking up leg room.

I can think of two vehicles where an automaker tried, and didn't increase the height much the car is recognizably the same - the VW e-Up and Smart ForTwo Electric - which I think are both bad range wise.

Times when the automakers also increased the height would be the Peugeot e-208 and Opel Corsa-e (same underlying car) and Mini Electric - but I think there was a lot of planning ahead in those designs to have platforms that could be EVs rather than retrofit of existing platform.