I don't know if all EMG pickups have ceramic magnets (don't think they do), but the OG P/J pickups do.
I've installed the P-46/J-45 in three basses and neck/bridge J-45 (x2) set in one bass. They are a bit bigger (at least the P-46 was -- maybe the J is, too) than the EMG set they replaced in my main bass (a Fender Power Jazz Bass Special):
- In the Power Jazz Bass Special, the P-46 required a little sanding of its plastic case to fit the neck pickup route properly and adjust up and down easily (I did not want to mess with the route on the bass). The J-45 did not (though might be a tighter fit than the EMG was).
- In the fretless Franken-Squier P-bass (now P/J) in my avatar, neither pickup required any modification to fit. (Maybe the P-46 was a hair tighter in the pickguard than the stock pickup? Still adjusts fine.)
- A Squier P/J that I gutted and modded for a friend required no modification to install the pickups.
- An I-think-Warmoth-bodied, 8-string Jazz bass that came to me with EMGs likewise required no modification of the pickup cases or body routes to install the Wilde pickups. (The pickups work great in the 8-string, by the way -- the close-spaced poles do seem to behave at least somewhat like a bar magnet -- but 5- or 6-string applications would depend on the placement of the outermost strings vs. the sensing area of the pickups -- might be worth asking Wilde about.)
Andrew Drake routed for and installed the J-45 in my fretless bass -- as far as I know, his usual template worked without trouble, though, when I asked him about the fit of the pickups (vs. the EMGs) in my Japanese Fender, I think that he remarked that, while you'd think they would be, Jazz and Precision pickup dimensions are not 100% standardized.
That was awhile ago -- and I can't speak for Andrew -- but, if you're concerned, I'd contact Wilde pickups and ask them for the exact dimensions for the bridge position J-45 (they're given for the
P-46 -- Width 1.10",Length 2.24", Depth 0.82" -- but I didn't see them for the Jazz pickups).
EDIT: Between the EMGs and the Wildes, I had a set of Reed James pickups in my Power Jazz Bass Special -- at least one of them (maybe both) required a similar slight sanding (well, moderate sanding with a slight effect) of the plastic case(s) to fit easily in the bass -- maybe its routes are a little smaller than normal -- or maybe EMG pickups are -- or the Wildes a little bigger than normal -- or a combination of the above. Can't tell you for sure, but someone in the Luthier's Corner might be able to (or you could look up dimensions on a few J-bridge templates -- or measure the current pickup and/or route in your bass).