Batboard

I was wondering if it would be easy to convert any of the white PS mobile mics for a 4 pin CB (I have the pin out for the CB end of the deal). Could it be as simple as finding an older one with similar guts and splicing inside the Moto housing, or would it be way easier to just transplant the guts from a run of the mill CB mic into a gutted Moto housing?

A Motorola mobile mic will be a "carbon equivalent" (DC biased, low impedance) where a CB mic is typically a dynamic mic. You're probably better off transplanting the mic cartridge from the CB mic into the Motorola housing.

Do you know which model Motorola microphone, or what model radio it comes from?
There are some significant differences from one type of Motorola mic to another that would affect what your options are.

That's the thing. This is a pet project of mine, and unless my old Spectra W9 mic is still at my mother's after a house fire and three moves, I'm literally starting from "if there's a better one to buy, which one is it"? I know from browsing and dreaming of getting a ham license and putting a radio in my rig that there are loads of model numbers for these things.

I have a Motorola GM300 mic on my ICOM7200 HF rig.

The biggest problem would be the PTT switch.
On a 4 pin CB microphone the pins are usually something like:

Mic audio
Ground
Transmit
Receive

The PTT switch is a DPDT switch, two sets of contacts, each with a common, a normally closed and a normally open contact.
One set of contacts switches the Mic audio, the other set switches Ground between Receive and Transmit. This is different from how any of the Motorola mics work, they only switch Mic Audio and Transmit.

The Spectra mics have the required DPDT switch, but it's on a circuit board with the mic element and amplifier. Older mics, such as from a Motrac, have DPST switches, two sets of normally open contacts, so they'd be missing the normally closed contact required for Receive on a CB.

Then there's the problem of the mic element. You either need to find some way to interface the Moto circuit with the CB, or hack a CB element into the housing.

Another possibility is to build an interface box to go between the mic and radio that would provide bias voltage for the mic and a relay for TX/RX switching.

Sorry to say, it's not going to be a simple matter of changing a plug or cable.

Sounds like fun, if Radio Shack were still around to sell me parts. I would ask you for a schematic of what the box would have to be but how much of a pain would it be to get the required parts?