The Gen1thou shock doesn't exactly have a great reputation.
The big problem is the spring, for some strange reason Yamaha gave it a spring that might be ideal if you weigh about 8 or 9 stone, but yeah most of us weigh a bit more.
Not long after I bought mine I had the pre-load turned up to 10, and I'm only just over 12 stone.
It was OK for 10,000 or so miles but then it gradually turns into a pogo stick. The damping just gets shot with too much work from the weak spring.
So if you can find one, you'll probably find it's shot. If you are lucky a wee damper overhaul - ie new seals and a re-gas - it might be OK, plus of course even with the 600 you'll probably need to fit a new spring.
The K-tech upgrade (don't think it's available anymore) cost me the best part of 400 quid, you end up with a top notch shock. But all I have is a box of bits left over.
Maybe try asking in the thou section - a lot of em don't look at general.