Earlier quoted context omitted.
I really wish these options would become more common, maybe even similar to the whole ecosystem around old American muscle cars. Pick and choose a (mostly) standardized drive train, standardized battery packs of different capacities and and your electronics to connect everything together.
I thought the issue is more the batteries and weight distribution. That is, EVs usually have the batteries along the length of the chassis to spread the load out, which is not practical to retrofit onto an ICE car. And if you fill the engine bay on an ICE car with battery packs during the conversion, the weight distribution will be extremely uneven and cause trouble with the suspension and related components, poor ha…
That's something spewed by people who don't know enough about cars to know they're chasing the wrong criteria. Battery placement is like a 2nd/3rd/4th order problem. You could fit a very respectably battery in the space where the fuel tank and exhaust go and if not there then the floor might just have to get a couple inches taller in the rear row. Not a big deal. Making battery cases to fit those locations is hard, but also not crazy. Just scan it like Weathertech and Uhaul do for mats and hitches.
The first order thing that's keeping all this from happening is that there's no money in it after all the expensive re-engineering and low volume manufacturing you'd need to do to integrate it into the vehicles you want to support.
This is why the industry is kind of stalled at the "supporting DIYers" level. It just don't work without free labor doing the vehicle specific bits.