Oh you're right; I must've gotten lost in the sea of equations
So Idss only affects the value of the S/D resistors (while keeping the ratio) and the current draw of the device. Does that mean I have (and only have) to change those resistors to make the 5457 sound like J201, given they have the same Vp?
Assuming all other things are equal, if you double Idss you halve Rd and Rs but it's not really the whole story.
Even J201's vary from unit to unit. The great majority of DIY circuits using J201's have an adjustable Rd.
The adjustment lets you set the drain voltage, which is one of the more important aspects for the right sound.
The Rd adjustment allows you to tune out most of the variables at once.
[Edit: However, the result usually has a different gain.]
The point where you get to tweaking Rs is when you realize you can't making everything equal with different JFETs.
If Rs is not bypassed by a cap, and you want the same gain, then you can tweak Rd for the correct gain.
If Rs is bypassed by a cap then very roughly want the DC voltage across Rs as least as big as the circuit you are trying to match.
[Edit: Sometimes you can keep Rd the same and adjust Rs to get the correct the source voltage. This method can often give a better gain match.]
The easiest way to do it is try a few Rs values, (accurately) adjust Rd so the DC voltage on the drain is right, then choose the one the sounds the best.