I've owned a couple of v-twin Guzzis, also got to drive an ersatz Morgan powered by a Guzzi twin up front in place of the standard JAP. In spite of being a tad... eccentric, the motors are quite solid, and the 90 degree setup cancel part of the primary vibration with the secondary from the other (and vice-versa) so it's quite smooth for an engine without counter balancers, but the crankshaft layout lends itself best to a rear-wheel-drive vehicle, and I think that Fiat might have been FWD(?) which might explain the lack of follow-up.

It doesn't take a lot of power to move a mini car around. I had the Honda 600 (FWD, twin-cylinder air-cooled 600cc engine and it had plenty of go - for a car. Get used to a motorcycle and nearly all cars are desperately slow in comparison.

All that said, I suspect the Guzzi photo (the one of a girder fork) is actually of a Guzzi single (the one with the external flywheel). That one had a horizontal cylinder, no counter balancers but wasn't a boneshaker like, say, the Velocettes made about the same time. I was on an AJS/Matchless G50 at the time and pretty inured to vibration...