Don't know how to change the rear gear. Not enough information. Is it an Eaton 172 rear, or something like that? Once you ID the rear, you can have some luck getting new gears.
You could always change to larger tires. 11 R 22.5 are pretty tall. That'll speed you up.
As long as you don't carry any water (I'm assuming this is for show) you'll be fine. Otherwise you might not have enough torque to move that beast.
