For some reason I was convinced the original pickups were single coils, but yes, the 501/1 used several varieties of mini humbuckers over the years. D'oh! Sorry for clouding the issue.

Though if you really wanted that 500/1 sound, getting some Hofner staple pickups would be a good first step. I'm sure the kit mini humbuckers will be more powerful, have more windings and so have a higher inductance and sound very different.

You can always fit 500k pots see how they sound and then try wiring a single 240k resistor between the common output signal and ground, which will give almost the same effect on the sound as having 250k pots. You can then either leave it in place or remove it. Easier than changing pots.