engine inter change

Hi Rhal.
Got a 'picture window' in the motor, huh?
If I were replacing the motor, I'd put the same hp (15.5) back into it, and you wouldn't run into any bolt hole problems. I think if you went down in size, you'd have a different mounting bolt hole pattern. Going up,don't know.
Going up in hp, don't know what you'd bump into, never done that.
Any re-wiring wouldn't be that big a deal, if you know what you're starting with. The electrical connectors may have to be removed (chopped off) and you may have different color wires, but if you know what was there to start, and what the new engine has, it shouldn't be that big of a project.
Now, squeezing in a bigger engine into the older engine space might present a challenge. But, with patience, a hacksaw and welder, and ingenuity, most everything can be made to fit into most everything.

Without being too specific - a WIDE variety!

The engine mounting holes are pretty universal. You may need to fuss around with fuel line length, wiring connectors, exhaust, and throttle cable length or configuration, but usually these issues are fairly minor.

I am in the process of replacing a blown Briggs 18hp Intek OHV single with a good Briggs 19.5 hp horizontally opposed twin. Luckily for that I have the donor tractor for the muffler and some miscellaneous brackets and etcetera, but it bolts right up with only a little bit of interference between the engine and gas tank and I can remedy that easily.

You should even be able to put a Kohler or Tecumseh engine in with little difficulty. Keep your eyes open for a lawn tractor with a blown transmission or rotted out deck - might be cheaper and easier in the long run to pick up an entire donor tractor then sell, give away, or scrap what is left when you are done.