You can buy Koso gps attachment - they are to fit ALL Koso gauges. I'll be using one on my slabby with my rx2n gauge. Much less faffing than trying to calibrate the speedo.
see link:
http://kosonorthamerica.com/shop/motorcycle-mx/gps-speed-signal-converter-kit/
May be worth giving them a bell. I think Gary Inmans Black Arrow was similar spec to what you've described....
That bike was built very closely with Racefit
http://s188.photobucket.com/user/guygilbert/media/C404A324-EF85-417A-8A91-C3D1048FFB54_zpshzc65ikl.png.html][/URL]
Looks like @Zepps one does too. In one of the photos at top of this page you can see the plate with two holes in in, attached to underside of the beam- to take to drop down mounts