Without seeing the schematic, I presume that it does need up to 250K resistance to provide the correct amount of distortion. You can use a 500K pot with a 500K resistor wired between lugs 1 and 3 to provide the 250K resistance. (The taper would change a bit.) Or you could add a 150K resistor in series at lug 1 of the B100K pot. This would give you 250K at the full CW rotation but only 100K at full CCW rotation. This would limit the amount of control you would have.
I looked at the website for PEDALPARTS Australia and They are showing several types of 250K LOG pots (250KA or A250K) as well as 500K LOG pots.