If you have faith in your luthier's skills, then just leave the job in his capable hands. This kind of thing is absolutely routine for any experienced guitar tech or luthier. Piece of cake.
As for the slight difference in the dimensions of the Lawrence "P" pickup, your guy would likely just carve out the cavity a bit, so as to fit the new pickup snugly - especially since your Ibanez is a player - not a museum piece. Much
easier than fooling around with putting the new pickup into the old housing - which IMO would be just asking for trouble.
As for your "wiring" concerns, I'm not sure I follow. If you're looking for a simply passive wiring job, there's nothing to it. But if you want to start using the stock preamp (a good thing to have), then your guy will need to do a little troubleshooting. Again, it's absolutely routine for a job like this.
The solution could come down to something as simple as reconnecting a loose wire, or something much more extensive. It's really impossible to speculate from a distance.
If you really want to preserve the active electronics option (I certainly would), then you might even consider having an updated onboard preamp installed in lieu of the original. While I have no idea of how good the 1982 Ibanez Musician preamps were, I can tell you that the state of the art has advanced dramatically
during the past 30+ years. You owe it to yourself to at least consider what an upgrade could do for you.