Thoughts? The guy says it's 43mm dyam.
He probably means the piston diameter is 43mm. That's the same as the 800/620/695 et al. The outside diameter should be 50mm, and for the GSXR 600 that is true for most (but not all) years.
You will want a few more measurements.
measure the overall length of the two forks.
compare where the forks are pinched by the triple clamps
what brakes will you use
how will the front wheel be aligned
how will the brake calipers be aliigned
what fender will fit
will your stock brake lines work
start asking the questions before buying.
Thanks for the advice guys!