Dave... The problem is that the Honda built a Spree to conform to US "Moped" laws... 49cc, 1 speed, 2HP, 30 mph... So Honda made the complete engine system to conform to that. (though i think a Spree shows 32mph on the speedo)
... Yet it would be just as easy for the manufacturers to make a 49cc, 1 speed, 7HP, 50mph engine system.
But if they do... It has to be licensed as a motorcycle, and ridden (by law) only by somebody with a full drivers license and motorcycle qualification.
Yamaha, Derbi, and Aprilia already sell them here.
But for you to turn your Spree into a 7 HP 50mph bike... You would need to re-do the entire engine system ....Air filter, carb, reed valves, cylinder, piston, head, exhaust pipe, and muffler.... Also, gear the "torque converter" double pulley system up higher.
The easiest way to faster is to get a tuned exhaust pipe for it (if they make them)... But you should be aware that it will only go faster by revving higher... And I broke a crankshaft doing just that.
Some of the Euro manufacturers (Tomos in particular) have produced peds that exceed the Government rules.... I don't know how they did that, but it doesn't bother me.
I'm betting the guy who asked the question about a 55 MPH Yamaha Slider is not in the US.