There are a few different things you can do.
You could measure the resistance from the wiper to each side lug of the pot and then replace the pot with matching resistors, but this only works if you can come up with a resistor combination that is close enough to what you need to work.
You could also use a multiturn trim pot, which provides much greater accuracy than a regular pot, to set the initial value and then put a dab of nail polish (I'm not kidding) on the shaft to hold it in place. Nail polish is fairly easy to remove if you have to adjust later on, and it looks absolutely fabulous.
Regards,
Torben