Can I offer an alternate product suggestion?
The i3 platform has a reasonably large amount of space allocated for the REX engine/generator unit. On BEV i3s, that space is simply empty. The worst kind of empty too, because BMW didn't even let you use it for storage - it's just open space underneath the car.
If you could manufacture a supplementary battery that fits in that space, which works in conjunction with a factory battery to increase the range, you'd really be on to something. It'd be smaller, take fewer cells, and be more widely applicable than an entire main battery replacement.
An electrically different option could be a battery-based pseudo-REX - a drop-in replacement battery pack that pretends to be a REX to fool the computer. Then anyone who has a REX engine failure, or who just wants to change to a BEV, could remove the gas engine, install the new battery unit in its place, and enjoy a few extra miles of all-electric range. The car would just think it still has the REX, and switch it "on" when the main battery pack was low.