Your best bet is to fit a Sigma BC906 bicycle speedo onto your scooter.
I placed the magnet from mine on the inner ring of my front wheel, extended the wire by soldering in a patch wire so I could mount the sensor on the original speedo cable (so it's in range of the magnet), and mounted the gauge itself right up on the original cluster using some high strength double sided automotive tape (the kind you use to attach wind guards around windows and such).
Then I later took the light out from where it fed in to light up the original speedo in the cluster and extended the wires on that and mounted it to a bracket so it would shine down onto the Sigma gauge (doesn't have an back-light). That allows me to see what speed I'm going at night without needing to drive under a streetlight or something.
Once I had it in place, I calibrated it based on the number of millimeters the scooter travels to make one full revolution on the front wheel by marking the floor and the wheel and then rolling the scooter with me on it (since my weight compresses the tire slightly) and had a friend of mine mark the floor where the full revolution stopped.
The reason I recommend the Sigma is that it's only about $20 and can read accurate speeds up to 185mph. I know... that seems horribly extreme for a scooter, right? Well... I found out the hard way that there are a bunch of other bicycle speedos that will start counting backwards after you get passed 45-50mph (most of them actually). So you get up to 45 and then you speed up some more and now it says 43mph! The Sigma BC906 doesn't do that... it'll accurately display the speed faster than you could ever want to take your scooter (unless you're insane). heh.
Other perks about the Sigma BC906:
1. Constantly calculates your average speed and even tells you when you're going your average speed, faster, or slower than your average speed (little arrows and a square signalling which you're doing).
2. Automatically records your highest top speed (mine currently reads 51.28mph)
3. Gives a VERY accurate speed reading if you calibrate it right. I tested mine vs. a GPS and it was dead on accurate and showed more variance than the GPS even (decimal places on the speed where the GPS dealt only in whole numbers).
4. Has a trip odometer and a total odometer
5. Has a clock
6. Big easy to read numbers showing me the speed I'm going while displaying one of the above extra info below the speed (has one big easy to press button to switch between secondary functions).
It also will display either MPH or KPH... but I figure you'd just use MPH anyway since you're here in America.
So yeah... that's what I would do if I were you. I just use the cluster for Fuel and RPM now. Well worth the investment.
Oh yeah... hehe make sure you leave a good amount of slack for the wire to extend passed the shocks travel distance. I made the mistake of not giving it enough slack (had it zip tied up with not enough play) and it broke at the solder point and I had to re-solder it. So... just something else to keep in mind if you do it. I ran the wire up through the front panels so I don't have some ugly wire just sticking outside the bike's body and such.
And actually nearly every single motorcycle or scooter speedo will be optimistic. They make them that way on purpose. They all tell you you're going faster than you really are. The only way to get a truly accurate speed reading will be from a GPS or a well calibrated bicycle speedo like the Sigma.
A lot of motorcycle owners actually use the Sigma on their bikes as well because it will give them accurate readings.
Hope that helps.