What problems are you experiencing? The volume knobs all seem to work fine on my PSR600's and PSR500. The PSR500 is a little scratchy sounding only when adjusted however it was that way new.
Finding another pot to physically fit may be a challenge.
A series, or even parallel resistor might help.
I have a PSR500 and Pro106, both pots are just a bit "scratchy" from old age.
I'm curious where you sprayed the cleaner as there is no opening into that pot. Did you use a knife to make a small hole in the plastic to create a hole in which to spray in the cleaner?