Simple solution: Take your PTO generator and a 1000 rpm electric motor and run the grass cutter off of electricity. Best if your genny has 3 phase outputs. If not buy one that has it with the money you are saving. Cheap 3ph electric motors are all over the place. Try not to run over the electric cable you have to run to the cutter from the genny, you could start the field on fire. Even better would be if you have a front pto and run the genny from it. You need a longer cable though, and pushing a trailer in a field can be tricky.
Another possiblity would be to use a 1000 rpm DC motor and run it off of a battery trailer. Add some solar cells and you might be able to sell power back to the electric company on a very sunny day.
I suppose you could double the length of the mower blades, too. That would get the blade tip speed up but raise heck with the mower case and shroud for the first few revolutions. (Use ear muffs and a good radio).
Why do you have to actually run it ? Just try dragging it across the field (take the wheels off, off course). It takes work to cut grass over a period of time. Work is power x time. And power is the integral of torque x rpm. So, for 1000 rpm vs. 540, you can still use the same amount of sunlight. (BTW: 1000 is not twice 540. So the extra 80 rpm from a speed doubler could cause catastrophic failure of your sense of humor).
