I can't vouch for quality, nor can I direct you toward a Yamaha R1/R6, but I can provide a few links:

the Escapist's Poser Props
SHAR3D
The 3D Studio
Honda Fan (parts in .lwo and .dxf formats)
3D Commune (hdrider's Harley-Davidson Flathead in the Poser Free Stuff)

Here in the Marketplace, you can find a futuristic cycle by Ra Graphics, a motocross bike by Sprinter and goldtassel, and a custom bike and parts by billy-t.

Over at RunTime DNA, Traveler sells the Freakin' Beast, a Giger-ish chopper with optional sidecar.

DAZ 3D brokers a Harley-Davidson by Patrick Martin.