To start with here's two articles from the archives:https://community.guitartalk.co.za/d/26755-tools-for-the-task-p-90-pickups & https://community.guitartalk.co.za/d/26979-paf-101-what-made-seth-lover-s-pickup-design-so-innovative
P90's don't quite sound the same in a humbucker size - something to do with the physical dimensions adding some 'magic' to the P90. Though that might just be your mojo!
I've played a set of SD Antiquity pup's which had surprisingly good cleans - good cleans in a humbucker usually means alnico magnets and lower outputs (8k-10kish). I personally think coil taps generally don't work well on humbuckers (I tried a Suhr SSV that tapped great though).
Do have a look at filtertron's - TV Jones if you wanna break the bank, Gretsch or even the one's that fender do - the fideletron's which you can source via Paul Bothners as a replacement part (Last I checked it was around R600/ea).
As for changing - two things:
1. pot values, humbuckers are usually 500k pots and singles (P90's) usually around 250k. The difference is the lower value the pot, the more treble they'll attenuate.
2. 50's wiring for humbuckers - the link escapes me, but it's googlable - this can help retain some treble when going with a HH config - also changes the way the treble bleeds when you roll off the volume - cool mod.
...and of course assuming the pups fit in the standard cavities/pickguard - though both can be fairly easily modded if motivated.