Before you do:
When was the last time you remember it working?
Can you feel it? i.e do you know exactly where it is?
Being an older implant it is less likely that it has migrated (moved) , but it does happen…
If you can’t feel it, you could try and locate it with a neodymium magnet and try reading again.
The size @Eriequiet used should be enough
unless of course you can still “see” it, or feel it.
I’m sure you are well aware, but try for perpendicular read to increase your chances.
Test your readers with your working xNT and try and use the best performing one for your non-working xNT
Let us know how you go
good luck