a clutch/tranny can last a long time providing you use a good oil, and you change it frequently.
typically if it starts slipping while under acceleration...meaning if it revs to the moon if you're in 3rd, 4th or whatever gear, and you're only going 5mph with no pull from the motor...then it's slipping, and it's time to inspect, or replace the plates. .
Pay attention to how much/little adjustment you have in the cable at the lever.
I just use OEM plates...but most of the top brand aftermarket ones will work fine.
You can get different springs with heavier or softer "pulls".
If the fingers on the basket are really grooved bad from the plates....you could file them smooth to get a little more life out of it...but a new basket would be in the near future.