If the engine is running hotter, I would suspect that the flywheel key may be sheared or possibly the cooling fins are dirty. I assume that it runs well?

***"Replaced the rusted muffler"***
Was there perhaps a heat shield that had already fell off the old muffler that did not come with the new muffler?
Or do you think it is the direct hot exhaust stream blasting the grass?
If it is the exhaust stream, you may be able to fit a deflector extension onto the outlet to change the direction of the stream.
If it is radiant heat from the muffler canister, there should be a heat shield in place.

tomplum - OK, I'll check out the cooling system. It seems to be running fine, is not overheating to the point of engine dying, or anything like that. Thanks!!

Mownie - It is the hot air exiting the muffler and going directly onto the grass, and it is very hot. The heat shield is there, surrounding the muffler on all sides but the bottom where it exhausts. I've thought about jury rigging a deflection of some sort - I'm certain nothing fell off though, I've looked at the parts schematic, nothing was there. Thanks! I may end up doing that at least on an interim basis so I can at least mow the lawn. And for those out there who are going "nooooooooo!" don't worry, I'll make sure that any grass clippings that get caught in there are removed regularly so I don't catch the mower on fire! :)

tomplum

Bingo! The small fans that are engine driven with a belt are not working.

Well, that certainly makes sense. The down draft of the fans does help disperse the exhaust stream (on a zero turn mower of this style). I just didn't think of that as a main culprit.