The ongoing acceptability of these combinations may be determined by future
legislation. Operators utilising these equipment configurations are urged to
update.
A fundamental tenant of ADS-B is that the GPS signal be from a C146a unit.
8.2.1 The ADS-B transmitter needs to comply with the minimum performance standards
detailed in RTCA/DO-260, DO-260A or DO-260B Paragraph 2.2 as appropriate for the aircraft
type.
If you look up DO-260 you will see that it specifies C146a.
There is a project running to consider allowing C129a, but I wouldn't hold my breath. I'd start saving.