Gearing?
Currently It only uses third gear with a neutral on either side.
Drive ratio to the ground?
No idea I'd have to find out what sprockets they are currently using for what track.
Top speed before we modded the transmission was approximately 90.
If we just wanted to see how fast we could make it, well over 100 with simple gearing changes.
Now that it only has 3rd gear, again depending on the gearing they are running I'm guessing about 50-65.
HP?
Not as much as it does now
I could only guess but likely 75ish at the crank.
The new top end will certainly make more, and if they wanted to change the carb, exhaust flange, and a few others things, even more.
We can easily give them more HP then they can use.
The real key is the torque values and at what RPM making the Kart easier to drive.
If bikes are for kids I'll never grow up.