Hey, I've a Wilshire too, with the stock Epi style mini-hum in neck position and in the bridge slot... a Duncan SM1.
About your quest: P90's have typically a very high inductance (often more than 7H = higher than a DiMarzio Super Distortion) and therefore a low pitched resonant peak. So I'd try something with these features.
In the Duncan "Pickups comparison chart", the SM3 B has the lowest pitched resonant peak : 4khz, close to the 3.8khz of a vintage soapbar in the same chart.
it won't sound the same than a P90 for questions of "magnetic windows" but it might exhibit a similar EQing curve if paired with the right components IMHO.
Side notes:
1) the stock bridge PU of my Wilshire is apparently wound with 43AWG and has the inductance of a full-sized P.A.F. style humbucker. Suggestively, it sounded largely like that to my ears when it was still in this guitar: I've compared it once with the bridge HB of a SG and there was not much sonic differences to my ears (that's why I've changed it for a SM1, actually). YMMV.
2)For a P-90 tone without hum, I've personally got good results with...
*Kinman P90Hx.
*noise cancelling air coils in the Ilitch fashion - easy to wind since they include only a few hundreds of turns of thick wire... less easy to put in a guitar since they need a wide area to be laid. My experimental guitar hosts one in a square routing under a giant backplate.
*a mere P90 without magnets nor baseplate, hidden in the electronic cavity and used as a noise-cancelling dummy coil. I've paired it with some LRC filter correcting the whole EQing and giving the sound of... a mini-hum.

Gibson did apply the same principle with a few twists: they did use regular P90 bobbins but with rod magnets giving a lowered inductance and more dynamics. Then they wired these "fenderized" P90's in series with a dummy P90 inside the guitar. If memory serves me, that's what one finds in the "Blueshawk" model...
Good luck in your quest.