shouldnt be difficult to get your speedo drive working though. if you use your stock sv650 front wheel, you have a 17mm front axle. so it will come down to which dirt forks you use. you could get the forks machined and threaded to just take the SV axle.
or if you are forced to use a smaller diameter axle, you'll just need to get alloy wheel spacers that are machined so that they can insert into either side of the front wheel and act as axle shims too.
are you sure a gps wouldn't count as a working speedo? most gps units have a speedo interface with a large speed readout and an odometer, often two odometers.