Looks like a clean survivor.

If your stock steels & fibers are within spec you can just put a set of 6 Barnet 85-86 OEM Suzuki LT250R springs in the clutch and it'll hold fine.

Another issue could also be the oil used, if it has friction modifiers it'll make the clutch slip.

I recommend Maxima MTL-R 80w. for the clutch/trans.