I recall this subject being talked about before on here and what i got out of it is that HP rating mean nothing. Isnt that why briggs got sued and is moving to the cc ratings and not using hp? its the cubic inches/cc's that matter.
I havent seen a 13.5 in person so i am only assuming they are the same. But i know for a fact that the 10.5 ohv and the 12.5 ohv are the same thing, or have the same cubic inches and block anyways...