All HST tractors use directional and ground speed controls that are independent of the engine speed control (throttle) for a reason...
You cannot stop the tractor with the gas pedal.
It is simpler to set the engine speed to a set RPM and then vary the speed and direction of a hydraulically propelled machine by varying the flow of the hydraulics with a pedal that is connected to a valve that can not only control flow rate but direction, too.
My Power Trac is a hydraulically propelled machine that I am very familiar with. It uses 4 hydraulic motors, one at each wheel, to provide propulsion. There is one variable volume pump as J_J described (he also has a Power Trac), that provides power to two series/parallel hydraulic circuits that drive the wheel motors (two motors to a circuit). There is a treadle pedal that controls both speed and direction. Left foot pedal is reverse, right foot pedal is forward. If you push the right pedal down, the left pedal raises and vice versa. If you take your feet off the pedals, the treadle self centers and the unit stops on its own. The farther down you push the pedal, the faster the tractor moves, but the torque decreases. So if you want more power, you let up on the pedal. Sounds weird, but that's the way it works.
By contrast, most HST tractors have an engine that drives a pump that provides hydraulic power to a hydraulic motor that turns a shaft on a transmission, which feeds a range case that feeds a drive shaft that turns a differential (or two if it is 4WD).
Either way, you have separate engine throttle and direction/speed controls.
Now, if you want to vary the speed of the unit with your gas pedal instead of a hydraulic pedal, you will need some very complicated and expensive hydraulic circuitry. The problems you are going to run into are going to be getting the unit to stop, change direction, etc...
How are you going to stop a hydraulically powered unit with the gas pedal? How are you going to change directions?
How are you going to start moving? There will be very little torque available if the engine is running at low RPM. If you step on the gas, chances are it will stall the engine before it overcomes the resistance of, say, a plow in a snow bank or a bucket in a pile of dirt.
If you set the engine at a high RPM but ground speed is controlled by the gas pedal, won't the unit want to move fast instantly? You will need a clutch to get it to launch smoothly and that is what you want to avoid, correct? See what I'm getting at? It is not an easy thing to accomplish.
As others have mentioned, reinventing the wheel is an option, or you can just re-engine an existing tractor. Just adapt a Corvair engine to an existing HST tractor.
Also, many years ago there was a prolific poster here on TBN named Mark Chalkley. He had a tractor called an Earth Force. It had what was called automotive control. As you stepped on the forward pedal, it sensed how much engine power was needed and increased the engine RPM accordingly. However, the forward pedal is not a gas pedal, it is a hydraulic pedal. It was a very fuel efficient setup and had the ability to run like I described, or he could manually raise the engine RPM for tasks that require high hydraulic flow but slow or varying ground speeds like brush hogging, mowing, snow blowing, etc...
Here is a link to how he describes it.
http://www.tractorbynet.com/forums/all-other-brands/24986-earthforce-ef-500-i-r.html
Good luck with your project. It looks fun. Corvairs were cool cars. My uncle was president of the local Corvair club for many years.
