Could be that the fork oil needs changing – do you know if its been done recently?
Or the fork seals have failed – make sure you have wiped dry the forks above the seals, sit on the bike, hold on the front brake and push down on the front 2/3 times to compress the forks as much as you can, then check to see if there is any oil on the forks. If it is wet then the seals have failed, if dry it could be that there is no oil left in the forks or everything is OK.
Here’s a couple of links on changing the fork oil/springs
https://gstwins.com/gsboard/forums/topic/past-it-mcc/6
https://pantablo500.tripod.com/id2.html
Some good info on here as well
https://www.twowheels.com.au/forums2/viewtopic.php?t=9420&highlight=forks
It might be worth putting in a set of progressive springs. I put these in the Kat and it transformed the handling (unfortunatly only from bloody terrible to slightly alarming).
Try these for seals and springs:
https://shop.wemoto.com/index.dyn?oid=1915434
https://www.mandp.co.uk/
let us know how you get on