The knobs have a part number, but the shaft is an internal part of the cluster.
Here it is in the parts catalog:
Here's instructions on disassembly if you need it:
https://986forum.com/forums/diy-project-guides/70976-disassemble-instrument-cluster-2001-a.html
I may have a spare cluster with the parts, but not a free minute to look until Tuesday. Hopefully you just need to slide the knob onto it.
Oh my.... just before posting, I figured I search the part.
Only $55 at Pelican. I've seen little plastic bits for aircraft that cost less. Maybe eBay or someone else on the forum can send one. I'll see if I have a few in stock on Tuesday. I might be rich!