New engine for Husky YTH20K46

I've just acquired a Husky YTH20K46, circa 2009. Overall the mower is in pretty good shape but the previous owner says the Kohler Courage 20 (SV-600) motor is blown. I haven't yet tried to crank it myself, but assuming it really is toast I'd like to replace it with something a grade or two better than the Courage series.

Any suggestions for what engine makes/models will fit with relatively little modification? Higher-end Kohler, B&S, perhaps even a Honda?