Most speedo`s are rated at 1000rpm at 60mph so any speedo in theory will do the job. There are plenty of outfits that can make up a mongrel speedo cable.
Edit. anything you do to the driveline bigger/smaller wheels,diff ratios and gearbox changes can change the ratio per mph and that is normally frigged by changing the plastic drive gears in the box or getting one of those in line adaptors. Heres a yank page of blurb on it but the same applies here.
http://www.partshp.com/speedometer_gears.htm
http://www.venhill.co.uk/Home are worth a try.
Another edit. Wheres my Cigar
Bob