The standard transducer for all three version of the 798 is the
XNT 9 SI 180 T. It is the Compact SI
If you just broke the mount, that is the MHX XNT.
As 44 diesel suggested, the XHS 9 HDSI 180 T will give it sharper images. The included transom mount is a lot stronger than the XNT mount which is all plastic.
This is a screen shot from a 798ci SI Combo using the HDSI transducer:
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1