Unfortunately, the Vanguard v-twins will not fit in my machine, too wide. They would've been my top choice. So, limited to a single cylinder my next top choice is a Kohler Command CV15 [C=Command, V=vertical shaft], or the Vanguard single cylinders [built before '96 or so].
There are several single cylinder Vanguards, 28Pxxx is [I think] the 14 HP, 28Qxxx is the 16. There is a 15-15.5 variant, not sure what their #s are, but as is the case with the flathead engines, the differences are external [carb or carb jetting] with the block, piston, rod, bore, stroke... being the same for both 28P and 28Q engines.
Because these 28P, 28Q are older ['87-'96], the condition [hours, wear...] will be harder to determine, so might just go for the CV15.
It'll all depend on what turns up.
The Hondas in this size are much less frequently available, and people selling Kawasakis think that they've got bars of gold, so are priced 2X what the Kohler and B&S [a guy wanted $700 for a used Kawasaki 15hp twin].
Hope this helps anyone in future searches for premium engine brand/model.