How hot the MOSFET should get depends on a lot of things. Do you have the spec or part # of the MOSFET you used to replace the one on the RAMPS board? Not all MOSFETs are good fits for this setup - you simply can't just replace one with another unless it's suitable.
Also, you should have a fan (eg: a small 40-60mm in dia fan) that is permanently on/running, pointing at your RAMPS board, as the MOSFETs, the PTC fuses and the stepper drivers all really require active cooling (ie: a on all the time fan) to be reliable.
With regards to the fan, I get a lot of people who say "but it works without it", but fail to realise that it's working NOW, which may not be later when the environment is different (eg: the room is warmer, different humidity, more/less natural airflow, board gets placed close to a heated bed, etc). RAMPS produces some amount of heat, and if you don't actively cool it, you can't guarantee that you're not going to have issues.
All that said, I suspect that your hot end has some real issues, as it shouldn't be drawing that much current.
