Summary/quick version:
Will the PiranhaMax series use the same transducer as my 597 did? I saw that it uses an XNT 9 28 T, and I think my 597 uses an XNT 9 20 T. Looks like the frequencies and beam angles are different. Is that going to make the Piranhamax incompatible with the XNT 9 20 T?

Are there any other cheap options you guys might suggest for either being plug & play with the equipment I have or being cheap enough to start from scratch with for just 2D sonar up front?

More details:
HB 597 at the bow has died. I have an HDS-5 flush mounted in the console and an HDS-7 at the console on a RAM mount that I can flip around when I'm fishing on the front deck. But I still like having depth at the trolling motor, which was all I really used the 597 for. So I was thinking I could buy a cheap 500 series unit on ebay (head unit only) but I just searched and was shocked at what those idiots are paying for stuff. I got my 597 on closeout in October of 2010 for $300, and I just saw a bid for an open box 597 for $385 on ebay. That unit has been discontinued for 5 years!

I still need to diagnose mine to see if it's the unit or transducer. But assuming the transducer isn't the issue, I'm in the market for a new unit.

Anyway, I assumed pretty much any 500 series unit would be plug & play with my standard 2D trolling motor transducer, standard power cable and 500 series mounting bracket. But after seeing the ebay costs, I started looking for a cheaper unit and that's how I came up with the Piranhamax 197.