Look at Woosh for a couple of crank drive kits: https://wooshbikes.co.uk/
Manufacturers from the list above. I think they usually have more choices but supply is short at the moment.
If you send them details (and pictures) of the bike they'll say if it is suitable and any issues you may have.
They will also provide generally excellent customer service before and after any sale.
As above, bottom bracket width is important.
One common issue is brazed gear cable guides under the bottom bracket; many BB kits fit snug under the bottom bracket.
Can be resolved if you don't mind grinding them off and rerouting cables.
You may have issues with front gear options.
Some people (including Woosh) have said that BB kits are even easier to fit than hub kits; as long as you have the tools and knowledge to deal with bottom bracket replacement.
If you are thinking seriously about this rather than just wondering, also look on https://www.pedelecs.co.uk/forum/forums ... ussion.42/