I having worked as a heavy equipment mechanic for over 35 years I would like to add to this. 1st point is look at what almost every piece of AG. equipment and almost every piece of heavy equipment in the world uses for power. Diesel for lots of reasons. What moves a piece of equipment? Torque and a lot of it. Here is where a diesel shines. The ability to run for extended periods of time at not just full throttle but at full torque output as well. And yes some gasoline engines come close to what a diesel will do, And the operative word here is close. Most notably would be the gas engines used in heliocopters in there early years. The drawback to those are the really short TBO's (Time between overhauls) 2,500 hrs at the most. And very expensive to rebuild to boot. Whereas diesels in the Ag and construction industrys often see 10,000 hrs or more before overhaul. Add to that the one factor that keeps diesels firmly in the equipment of the world is they are very efficient for the torque produced and fuel burned they are hard to beat. One type of engine that does do that (Beat the diesel) Is a gas turbine. And they can come in small packages with high hp outputs. But they are hugely expensive to build and to maintain. If that wasn't the case then why aren't they used more. They too have short TBO's compared to diesels and are probably way to complex to put out in the field plowing. Not to mention the dust would kill them real fast. I do have quite a bit of experience with turbines as I used to work for GE in their marine propulsion division. The perfect environment for a turbine in there isn't much dust to deal with. But they are hardly used in marine propulsion. Why too expensive to operate.
Number 2 Are you going to drive this ag. tractor with hydraulics? Have you computed in the frictional losses from that many hydraulic pumps and motors? If you are going to have a drive motor in each wheel then the losses would be staggering. I think that you would need closer to 800 of a 1000 hp to function. One thought that comes to mind and would meet most of your requirements would be electrical drive. That way You could have several small generator sets that for cost reasons and the ability to sync together and to have variable speed would almost have to be DC generators. With dc drive motors in the wheels. The real problem with either electric or hydraulic drive is the cost. Hydraulic motors and pumps are very expensive to buy and the so are electric motors and generators. So that leaves mechanical drive. Lots of chains and sprockets or shafts and belt's to drive that many wheels, That to sounds pretty expensive too. And very labor intensive to keep running. Have You ever been around a baler of a combine. It takes a lot to keep them running. Lots of gears and pulleys and chains and belts. And costs with all of them. And ultimately cost is everything because you have to be able to sell your product, unless this is just for you. I am NOT saying this to discourage you far far from it. Almost every successful invention ever made has come from inside the industry that it was meant for. Racing is a good example for the forum we are on now. Racers gave us seat belts, rearview mirrors, high speed tires including radials, dry sump oiling systems and many more. So I am not discouraging you at all. I am just trying to give you some things to think about. And one of the things I see is that you will almost have to redesign this to use a larger engine and it will probably end up being a diesel. I do know what I'm talking about here as I have successfully prototyped many mechanical products over the years including some electrical products. I have also wrote a regular advice column on finding someone to prototype products and how to do prototyping yourself for the web site American Inventor Spot. And that was part of the TV show American Inventor. I don't want to see you fail, but also don't want to see you spend money you don't have to. I really hope this helps Bill