The 20hp Honda will be more than enough with a 10" inlet hose, even with heavy and wet leaves - especially since you are currently solo. Really anything 16hp+ will do good with a 10" hose, of course the more the better! As I said before I've repowered our older Salsco units (which also have 10" intake but a 6" square discharge) with gx390s, and even with only 13hp they do just fine - nearly comparable to our newer 16hp/10" Salscos(Lesco). Only time it seems like more power would be beneficial is trying to suck up grass clippings, just have to back off and go slower.
Unless you find a better/cheaper deal, there would be little reason to go for more than a 20hp. Anything over that would provide no real performance change unless installed on a bigger machine with 12" intake and larger impeller (in which case 23-25+ would be ideal)
As for that kohler engine you found - that muffler setup obviously won't work. Will either have to cut/fab/modify it, or ideally just buy a muffler assembly that will work. These engines have been used in several brand truckloaders before, and there are mufflers assemblies available that put the 'can' on one side of the engine (side outlet) and route the piping over (which allows engine to be mounted flush or near flush to flat surface/bulkhead)
The 2 3/4" shaft length sounds short, though. I seem to recall most calling for ~3.5" shaft. If the shaft is too short the impeller will contact the bulkhead when fully seated. You can measure through the back of the impeller how deep it will slide onto the crankshaft. The engine crankshaft will often need to be about 1" longer than this.