I've been
told that manufacturer doesn't allow independent avionics technicians
to (legally) repair their products anymore for certificated
installations.

There are no independent avionics technicians. There are certified repair
stations who employ avionics technicians. If the repair station was doing
Narco repairs before Narco pulled the rug out from under all of them and so
still has the correct and current technical data, and some other celestial
bodies align properly, the repair station can still work on the Narco
transponder and return it to service. I know of at least two avionics shops
that can do this if asked, but they don't advertise it, and I think don't
really want to.

The original poster ought to send it back to Narco.