buy one with a salvage title... that just has minly body, firling and subframe damage... you're goingot crash it eventually anyway. Leave it uninsured and unregistered and for track use only. But race plastics, better brakes pads, SS lines, clip-ons, rear sets, safety wire it up and you're ready to race. Figure $1000 in plastics and parts to convert a 600SS for track us. Then consider $500 for revavling the front fork. and eventually $800 for a rear shock.
You should be able to find a salvage title R6 with under 10k miles for $3000. THis is what I would do if I ever wanted a trakc bike. For now if I do any track days, I'll just od NESBA and tape up my street bike and only ride it up to 90% of my limits for that extra margin of error... but fast enouhg for some real fun.