If you remove the pot, you'll be able to measure its resistance OR its value will be written on it as it will be a 'standard' value.
Once you have that you then have to do a little more measuring. I suspect it will be a linear value (where at half a turn its value is half, at 3/4 turn its value is 3/4) but it may be a logarithmic (or audio) taper if the values you measures are not as I've said.
Once it's out, then simply google search on sites like Farnells or RS Components and it'll show up so you can order & replace. FWIW, I'd spend good money on one (def not the cheapest) as they take a lot of pounding.
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