There's always something
With the Reliant-Guzzi package it's the alternator and distributor, and also the water and fuel pumps. Not so bad with the BMW, it's just the starter motor drive and it clears a main frame tube without assymetry - just.
There's always something that gets in the way when you use someone elses engine in an FF, apart from basics like the main mounting points and the exhausts being at the wrong ends of the lump.
Imagine, one day someone might actualy get to build an FF where everything is designed to fit...