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. [br]
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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. [br]
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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.