I have a tubeless set up but it looks like one of the valve cores is broken as its letting air out even when fully screwed shut. Is it ok to take a valve core out of an old innertube and put that in or are stans valves different for tubeless?

You might get a better answer in the Bike forum. LMMTFY.

I’ve swapped cores before. I don’t know whether it was just luck, but worked fine. Wasn’t Stan’s, but I wouldn’t be at all surprised if a core was a core was a core.

Prestwood cores are standard, I got a pair of cores only from eBay for a couple of quid if that.

should work fine. If it doesn’t, cut the whole valve out of the tube and used that. Works too.