37mm

if you can find a used set of 1st gen cbr600 hurricane clip-ons, they're the same diameter.

to put clip-ons on it, you're gonna have to move the fork tubes up thru the trees to mount the clip-ons above the clamp, or they can be put below the clamp, if you don't mind them being that low. that's how i had mine mounted on the 400.