https://www.nanoelectricbikes.co.uk/
This it the kit we used -excellent pre and post sale service by the owner (Lynda Power) and her team in the UK.
The kits took me about four hours to fit ( first one three hours, second one was much faster....see, I can still learn new tricks).
Tools for such as bottom bracket cassette removal are freely loaned by Nano if required. We have had no need for spares as yet but Nano keep a comprehensive stores inventory and are only 30 miles from us.
Swytch have a good website but rely on crowdfunding to support batch production of their units. Sadly, we heard of long waits for the product and little support/spares backup.