You likely have a Faria tach. If you have a tach but it's broken, Faria will repair it at no charge though you will have to ship it.
If you don't have a gauge, several online merchants sell Faria.
I have found a boat repair shop locally that has bins of used gauges and switches.
2006 Bayliner BR205
A1 Gen 2 1.47
5.0 carb engine