The Trig Avionics TT31 Mode-S transponder has been a popular direct plug play replacement for the King KT76A series transponders. A free EASA Minor change has been available that covers the replacement but has lacked ADS-B approval despite the TT31 ADS-B out capability. Just recently Trig announced that the already existing FAA approved ADS-B out STC has been validated by EASA (free for all Trig Avionics dealers). You will however need an approved WAAS GPS position source such as the Garmin GTN/GNS. Older TT31 transponders will also require a firmware upgrade.