The 17.5 foot header will be way too much in green stem beans. You have the smallest combine IH made at the time. They where made for a four row head and a 15 foot header.
You engine is running way too slow. That alone will really kill the power. Check the throttle linkage and make sure it is pulling the pump wide open. I have ran into that several times with the manual linkage that wear and get out of adjustment. So even thought you move the throttle to wide open the pump linkage is not against the high idle stop.
The engine should be a D-358. The early ones where rated at 112 HP and the late ones 124 HP. I have been told the turbo and manifolds off a IH 1206 will fit. You would have to ask a IH expert to variety that.
Myself I would not go to the trouble. The machine has other weaknesses that will cause issues if you turn the power up. I would get the engine RPMs up to where they should be and try that before I did any more pump jacking around.