Alvin,
I have two Comant CI-122 for Radio on the belly. Noteworthy about these; they use four screw to attach and the screw holes match the rivet holes for the reinforced plate. These are the certified antenna and Vans has the non-certified antenna which uses two screws.
For transponder, I am using Comant CI-105, again a there non-certified version is on the market and multiple brands exist. I am using one for both ADSB and TXPDR, since garmin does not require additional antenna for ADSB.
For VOR, I have installed a Archer antenna in the right wing tip. This is a home built and it is rather easy to build with super performance.
Both GPS antennas (650 and G3X) are installed in the wing tip as well. Of note, the BNC connector for 650 is a different type (screw in, rather push and twist) and I can remember part number now.
You can also install a marker beacon if you wish, few people now days install those.
Hope this is of any help.