They make 50 HP on the crankshaft/belt pulley, not on the powershaft. They are closer to 42 on the PTO.
Adding MW style pistons will not make it burn any more fuel than the factory pistons. If anything it should accomplish more work on the same amount of fuel because of the gained efficiency of the higher compression pistons. The bore and stroke remain the same which means the amount of air entering the engine remains the same as with the old pistons. Air fuel ratio remains the same as well, its just that you get more bang for your buck with the MW every time they fire due to the higher pressures when the piston reaches TDC.
My HP estimate is around 50 on the powershaft at the rated speed of 975. It will make more power than that at a higher RPM. This will only be achieved if the camshaft is up to snuff and fuel and ignition are fine tuned. You'll also likely have to clean the air cleaner screen mesh inside the oil bath air cleaner. They get gummed up over time.
Have that head worked with new valves and hardened seats. Have the block and head flattened and adjust the tappets just a couple thousandths on the tight side with new valve springs.
It'll still be thirsty but at least it will give you a few more ponies per gallon than the factory setup.