Very possibly but first check that it is for a hexagonal shaft.
So with VAT that is £26.34, they won't declare the P&P until you buy it, then add VAT to P&P. So that's going to be £30 just for a knob.
If that was the last thing you were stuck for before taking it on the road, it would be tempting I suppose. On the other hand in the present state it is in there are a lot of things that need attending to first. It might be worth biding your time & put the knob on the "things to look out for" list that you could pick up cheaper at your leisure.
Needle scaler is a good move. I also have a Clark but always forget to oil it. Make sure the grub screws are kept tight. The thread has worn out on one of the holes & the barrel keeps unscrewing itself. Unless you have a large capacity compressor use it for about a minute at a time waiting for pressure to come up to max again. It will be more efficient than using it until the needles are hardly move & then you have an even longer wait for pressure up. So time share the needling with another task to turn to when you start to lose a useful working pressure.