I have wondered what various motorcycle engines would be like.
A Triumph Rocket 3 engine in a Morgan 3-wheeler would be EPIC.
A BMW K1600 I6 as a transverse midship layout, with the center-aligned transmission driving a similarly centralized differential (not asymmetrical axle shafts like most transverse car engines) would be awesome.
Yamaha makes great engines, too. YZF-R1 has long been among the top supersport I4 bikes.
The only hurdle for aftermarket is engineering a reverse gear solution to suit... which a manufacturer would have much less trouble integrating into the design. Cars can't be walked backwards like a bike with feet on the ground.