Looks like a bit of room for "a" pipe, might could make room for 2 if you put cutouts in fairing kinda like they do to car hoods when a turbo is peeking through.
My question about a big single could have been better phrased maybe. I've been riding 2t 500's for the last 10+ years the vibes dont bother me , would it be worse or amplified in a street bike. I realize you'd need to gear it so it would be in the smooth spots at cruise RPM but...
An RZ350 motor would be good but I think you could get a better powervalve set up with something like the T-rex jugs as well as go bigger. No doubt a twin would be the smoothest, the banshee motors where barely bolted into the frame compared to a dirtbike single
Offset sprockets arent a big deal to shift the motor over to help with a kicker throwing the weight off center much would be a big negative though