Zarcan, is your governor working properly on your MH-50? Is the engine worn? ...out of tune? ...smoking? There are lots of reasons a 31 hp tractor might not perform at full potential, especially a 1955 model. If you want to compare the diesel to the gasoline engine you need to know the torques of both the engines at PTO speed. I always think of hp as the ability to reach a set rpm and torque as the ability to maintain it once you are there.
The diesel should have more torque, but the mass of the engine and flywheel may limit it's torque. Many of the old gasoline engines were pretty massive. If you can find the specs on both tractors, you can make a fair judgement. Otherwise, I think I'd ask the NH dealer for a demo. That way you would know for sure.
I used to use a gasoline Ford Jubilee (around 31 hp @2000 rpm) to pull a 5' rotary cutter. It was old and smoked under load, but it did just fine. I'd have to slow down a little in really heavy grass, but the governor worked perfectly when needed. I think your MH should handle the 5' cutter just fine.