There is a good chance that if the transducer is "old" it will be weak, and if you add a guage to it, it may fail. Also, you will need to know if it is a 50 or 200 mhz transducer.
If it is a through hull type (round) you might want to contact Moor Electronics in Buffalo NY...their digital unit with a new transducer is less than $250.00. You might ask Sailnet if they can get it for you. If not Moor has a website.