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Pickups come in all configurations, one of the more common types are P90 and Humbuckers.

This walk through shows the modifications required to a the cavity to convert from Humbucker to the P90

Time to repair approx 1hr
Length wise, it catches on the humbucker spring box recess.
Using the same mounting points for the pickup surround, we super impose a P90 cutout.

You can see using the original mounting holes, the P90 can be mounted at the original points.
To extend the ends of the Humbucker cavity we can just use a chisel, I prefer using a routing template, here I have a routing template made from 3mm mdf with a hole in it the same size as our P90.

Routing templates can be purchased online very easily from companies such as Stewmac, LMII, Allparts and so forth.
On the opposite side of my routing template I have attached a 3 mdf pickup surround, this allows me to elevate my template of the body and clear the existing pickguard and saddle mounts
Remove two screws from just one side.
Small router for template tracing.
With the router I just trim the side without the screws fitted.

Using a clamp also helps stabilise the routing template.
Refit the screws for that side and then remove the opposite side screws.
Remove the original pickup and assess the routed compartment for size / damage /shape.
The P90 easily fits in width wise, but will have a large side gap.
This is the schematic for a Humbucker Pickup Surround..
Using the 4 Humbucker mounting locations, we screw the template in place.
"How To"  Modify Humbucker to P90