When I fit V twins to mowers that used to run flat head engines, I often have to install them sideways because they foul on the hood.
Then it is a chop & weld jobbie on the exhausts.
On most side valve singles the exhaust blows strait out the side of the hood
On V twins it blows down & most have a purpose built muffler under the front of the mower.
A customer who drives big rigs, fixed this problem by fitting a pair of chrome stacks cobbled together from some old motorcycle exhausts.
He has a HD and gets std pipes pulled off new bikes really cheap.
The finishing touch was painting the hood yellow and sticking a Cat decal on the sides