I have fitted an Audiovox cruise control to my '84 1.9 Westy.
I bought it from JC Whitney for about $40 I believe.
Its not perfect but works great most of the time.
I could probably make it work better with a vacuum reservoir but I havent bothered.
My van is stick shift so it just uses a signal from the ignition coil as a reference signal, no magnets.
The control sits on the dash in a spare switch hole, bottom right.
It doesnt hold on a long steep hill but guess what !!! it doesnt create horsepower either!

cheers,  Steve Cotsford