You have a couple to options but since you mentioned "breaking off" the sonar TD, I would recommend the HDI 83/200/455/800 Transducer and then buy the Transducer Shield and Saver bracket to mount it to your TM. You'll also need 2 stainless ring clamps.
For the transome, I would get the TOTAL SCAN TD, and take advantage of sidescan. Plus it's 1 transducer that does it all: 2D, Downscan, and sidescan. You could swap the rear HDI to the TM, and add the TotalScan. Then you'd have the what you need.
The HDI will give you 2D sonar and Downscan together.