First thing to do is to check serial numbers and software versions. Some versions of that transponder need upgrading and Funke charge a few hundred quid to do it.
Secondly, any NMEA source should work. There have been a number of people who have used a PilotAware with a Funke transponder to provide ADS-B out, so, assuming (1), I don't see why the output from the PowerFlarm shouldn't work, but I'm unfamiliar with the unit and it could, as you say, be outputting a mix.
If you want a standalone one, you could do an e-bay or Aliexpress
search for RS232 GPS.
Or invest in a PilotAware!