I purchased this new back in the mid 80s and it was getting pretty rough with corrosion. Did a full cleaning and lube along with a new paint job. Just need to replace the drag knob (if I can ever find one). I repaired it with some JB weld because it was badly cracked. Currently working on two Sigma 2200ck 025s I've had since new. I had no idea how sought after these reels were until recently so I felt they deserved some TLC.
I mounted it on a brand new Ugly Stik Elite 7.0 med/lite and she casts like a dream.
Chances of finding just the drag knob are about slim to none. You'll probably have to buy the complete spool assembly. Both parts are obsolete. The 420SS will interchange. Both complete assemblies, 420SS & 430SS, are currently available on eBay. They're not cheap, though.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.XPENN+FISHING+REEL+430SS+SPOOL.TRS0&_nkw=PENN+FISHING+REEL+430SS+SPOOL&_sacat=888
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313.TR0.TRC0.H0.XPENN+FISHING+REEL+420SS+SPOOL.TRS5&_nkw=PENN+FISHING+REEL+420SS+SPOOL&_sacat=0