Is there a kit type propulsion system to add on to a existing foil mounted on a existing board ?
The flying rodeo motor system.
flyingrodeofoils.com
You need a separate esc/battery/remote for it but that’s the only one so far
Currently I’m selling the exact thing you are looking for. We always can talk about prices 
or you get the efoil upgrade kit from elevate.rocks.
It is a complete liftfoil motor unit with electronic box on top that you can mount on any Foil or windsurf board. So the only thing you have to take care of is the waterproof battery;-)