Mini speedometer and mounting cup/hardware from Dime City Cycles or similar metric parts house. GT550 tach cable. Close enough.
Bell 100 bicycle speedometer from Walmart, $20 or less, read the instructions.
Trail Tech unit like on my 2003. Read the directions. Dead on accurate, had a police friend pace me.
Air Something or Other (hard to find), self calibrates based on air pressure of the moving vehicle. Expensive, hard to mount, but dead on accurate. Look in a vintage airplane junkyard if you don't mind HUGE...
Or run your stock speedometer with the right sized tire, a 140/90-15, it corrects the built in "we don't know math" factor of running a 140/80-15 stock tire...