The PTO option is probally the easist and most economical. We are running one on the ChopZilla Hedging machine and it works great. The machine is amazing.
That is the problem. The pumps are mounted directly to the engine and driven from gears, finding a larger pump that would bolt up would be next to impossible. Then you would need to know if the existing hoses/hardlines, and valves could take the extra flow...
There are no aftermarket solutions other than a PTO pump or a belt driven one of the engine-both would require another tank, filter, etc. Basically a seperate hydraulic system.
If you are not using the PTO for anything, you can put as large a PTO pump as you tractor can support as in HP. Below pump, uses about 18.1 HP, and can pump 11.4 GPM