I never cut when grass is damp or wet. I’m cutting st Augustine grass and with the mulching blade I have to run in 1st or 2nd gear position. That mulching blade seems to suck up the HP. I really don’t let the grass get very tall before I cut. I guess I’m used to the Craftsman 46” with a 25 HP Kohler (with mulching blade Also) that I use at my camp.
Mulching actually uses more Hp than plain old throwing but less than bagging .
How much depends upon how much grass is kept in suspension above the blades
You will find a Gator® style blade will mulch better with less Hp drain than the std mulching blade
I fitted 4 high lift blades onto a 12-36 thinking it would throw the grass further
All it did was accumulated a massive amount of grass above the blades and bogged the mower no end.
IT takes a lot of energy to defeat gravity
As the mores get longer strokes, they get longer cylinders.
So the bolt pattern will be fine but the engine might not fit within the body work and the muffler might end up in a funny position.
A local make down here Greenfields decided to offer a bigger engine then found that the bigger engine would not fit in their frame so they had to turn it sideways so it cooks your left foot
Rover did the same thing but they turned the engine backwards which overheats the gravity feed fuel tank.
Sites like Repower , Small Engine Supplies & Small Engine Warehouse all have links to manufacturers sites so you can look up the line drawings of all of the engines, current & out of production.
Thus you can check the width , height & length of the engine you are considering using.