Ouch! That could be expensive, Pete's suggestion is the way to go, might be a while for one to come-up at a wrecker.
Pre-ST1300, I had a CB1300 with a notoriously inaccurate speedo. I fitted bicycle computer, calibrated it correctly, worked fine. It was a wireless Cateye unit, cost around $40.00. Had to modify the litte round magnet sensor to fit onto the hub, and a bit of fiddling around with the pick-up unit and an extension bracket on one of the suspension tubes, and after a bit of trial and error was successful and I was happy with the result.
A Google search on wireless Cateye computer and sensor will give you a better idea.