I use an after market system, 4.5lbs total,that can be moved to either of my bikes - or any new bike. There's a whole thread here on it: One motor/Shareroller drive. It's not one of those 50-100 milers, it's a super light friction drive that gives me a good 20 miles and that's all I need. Plenty of power, too.
I don't have the skills or the interest in learning how to build something from scratch, although I admire those who do. I have other interests that occupy my time with more priority. I hope we will see the emergence of shops that will rebuild/reuse e-bike components rather than have them be disposable like so many electronics these days. I've seen one or two battery rebuiders online but that's about it.