Don't use the pro-control as they just ground to earth....like switching the engine off and on again....fine for a computer but not a hi-reving VW motor
I've been using these
http://www.etcrevlimiters.net/2008/08/magneto-rpm-swtich.html we put one on Lee's Dauphine to make boost on the line...worked a treat and Hank is a nice guy....give him a call to get the correct model. You need the dual range one...they're not cheap but nothing that's any good is......Good luck.
Ian