Being the king of cheap, and having plenty of working motors around, I swap electrical motors constantly. To be honest with you, and in my experience, the 3/4 hp motor will not have any more real working power, if anything less.
The old motor was ½ hp, but was heavy due to more reasons then just the end caps. The equates to heavier shafting and overall, more rotating mass. Now you are getting into the realm of the flywheel and kinetic energy. (Trust me, as a snowmobiler I understand the inportance of rotating mass, you would not believe the difference on clutching that was needed just to add 1 pound of studs to my snowmobile track.
The old, heavier electrical motors also dampened vibration better due to their sheer mass, and vibration sucks up power, or at the very least fails to transfer it through the belting and pulleys as efficiently.
These are just real world experiences. Going from heavier, lower hp motor to newer lighter motors was always a disappointment. There are a ton of things I might have messed up on too though such as pulley sizing and ratios. I would still go ahead and try it. None of my power tools are not working because of a swap, I just noticed they did not work as well as they did before.
I have no intention of traveling from birth to the grave in a manicured and well preserved body; but rather I will skid in sideways, totally beat up, completely worn out, utterly exhausted and jump off my tractor and loudly yell, "Wow, this is what it took to feed a nation!"