Perhaps the simplest way to increase your ground speed would be a simple as changing the size of the sheaves (pulleys) between the engine and the inputs of your hydrostatic drives.
You likely will not find the parts from Cub without a lot of research, but you can likely find what you need from any industrial equipment supply store like Graingers or IBT (Industrial Bearing & Transmission). They all have good mail-order capability so you can search online.
I don’t know the exact layout of your machine, but all zero turns drive a hydro for each wheel. You likely have a single V-belt on the motor output haft driving a belt that runs to the input sheaves of both hydros. My 50” ZT1 has about a 4” diameter sheave on the motor that (I think, because I haven’t crawled under it to verify) drives both hydros.
Going from a 4” pitch diameter motor sheave to a 5” will increase your ground speed by 25%, and that’s a lot! It will also REDUCE your torque to the wheels by 20%, so if you have a lot of steep hills to climb, consider that as well).
Depending on the belt tensioner setup, you may or may not need to get a slightly longer V-belt as well.
You will need to know the motor shaft diameter and keyway size, the belt width (should be in the belt description in your parts list), either the Pitch diameter or Outside diameter of your motor sheave, and the WIDTH of the motor sheave hub (since there are almost certain to be multiple sheaves on the motor shaft - the second to drive the blades - and you want to keep everything lined up.
If you can find a suitable sheave - they’re relatively cheap - this is likely your lowest-cost solution.
Caution, don’t get carried away with speed - the small diameter tires on these mowers will accentuate any roughness in your yard - you don’t want a bucking bronco!
Good luck to you, and please let us know what you decide to do - and how it works out!
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