Welcome Sam

What type of bike are you looking for? If you're a bit more specific (size, style, brand), I can direct you to a couple of places that specialize in them. For example, there are a few dealers near the Lido area (North East 4th ring road) ... Harley, Suzuki (large bore) and Benelli. The Goldenport raceway (on the way to the airport) has the Ducatti, Jialing and MV Augusta dealers.

Jim (bmwside) specializes in sidecar CJ750s with BMW engines.

To acquire a legal plate, you need to either:

1) purchase a new bike from a dealer, and the dealer will make the plate arrangements; or,
2) purchase a used legal bike that is already plated.

If you are new to China, you may wish to secure the assistance of some new friends who can help you out. Acquiring a bike in Beijing can be a little complicated for the uninitiated.