Customized how? The face, or how it operates?
just get a garmin etrex gps and mount it cost ya 75bucks , will be accurate and you can use it if ya get lost
on craigslist, probably about 75 mph "all day long"
1992 Eliminator Daytona 21'
You may want to look into some automotive restorers that rebuild gauges. They could probly add a custom face for you. I seen a Pantera that had the MPH changed to Mach numbers, heh heh, that was kinda cool...
You're looking @ 50 bucks for the most basic, POS speedo.
For $75, you get ALOT more, fewer mounting troubles(Velcro it down), and it's actually accurate.
I've skied behind a 13' whaler w/ a 40, and it wouldn't go my normal skiing speed (32mph) while i was behind it in flat salt water (salt water is more dense, less prop slip, and more boyancy).
Try a speedo out of a tournament boat. Maybe you could find a used one. The range is closer to what you need (they top out at 40 or 50). Just a thought. Sounds like a wild ride