This is my experience using Dapol NSPARE9 "Magnetic Coupling Conversion Kit For Non NEM Pockets". It replaces the existing coupling mounting on N gauge stock with a fitment that will (should, at least) take any NEM-standard coupling, not just the magnetic couplings. Depending on what you're converting it may require cutting the chassis or bogie so it isn't something that can be easily reversed to return a model to factory condition.
The instructions that come with the kit are easy to follow. You do need some good, sharp knives, files and tweezers, a pair of tweezers that spring closed are especially useful.
First under the knife was a Peco wagon kit, purchased so I could practice without destroying anything cherished or expensive. The conversion pack includes a jig to ensure correct alignment, which works well with a fixed wheelbase wagon. I didn't photograph the conversion but here's the result:



I will paint my wagon at some point but I wanted to get the conversion right.

Converting the van went so well I decided to have a go at a Dapol Collett coach. The jig wasn't quite as helpful this time but it looked like the NEM mounting box would directly replace the existing coupling.
This was my first attempt, slightly too high but not so much as to impair operation:

The conversion process went something like this:
The bogie, removed from the coach, axles removed along with the brass spring that supports the Rapido coupling.


The existing coupling sliced off

The area made as flat, level and square as possible

Superglue was used to attach the new mounting box



The bogie re-attached to the coach, looks like a closer match to the correct height

First attempt for comparison - hard to tell because of the angle.

The NEM pocket itself clips into the mounting box and then a NEM Rapido, knuckle or magnetic coupling can be fitted as required.