[Smaug]

Suzuki Burgman 650: No longer made, but it was definitely the quickest scooter made in North America. Complex, electronic drivetrain though.

Honda Silver Wing 650. These were made until 2013 or so. They cost $10k new, but now really nice clean ones go for $3k or less. I had two of these; they're GREAT scooters. Off the line, they're not super quick, because the centrifugal clutch slips until 15 mph. After that, they're quite quite. (though it wouldn't keep up with a 1200 Sporty) They're not 100% step-through, as there's a tunnel that's needed for the gas tank and for fore/aft frame stiffness. These bikes are heavy, but the center of gravity is low, so you don't feel it while riding. Surprisingly easy to turn at parking lot speeds, due to the relatively small wheels. These will go over 100 mph, and they're a twin cylinder, 582cc liquid-cooled. Spark plugs, oil filter, air cleaner and drive belt are easy to get to. Valve check would
be a bitch, though. This would be my recommendation.

Burgman 400. I had one of these in about 2007 or so. Also a great bike. 400 thumper, so it's running out of steam after 70 mph. (though it would probably go 90)

One that really surprised me was the Piaggio BV350. It's quicker off the line than the Silver Wing, and quicker up to maybe 60 mph or so. Very stable, but not as easy to maneuver at parking lot speeds as the Silver Wing. (due to its big wheels)

Let us know what you go with.