OK, don't put a lot of faith in the gauge on the dash when it comes to voltage. The only place to read charge voltage that you can trust is right at the battery terminals. Boat volt meters are notorious for reading low.
You likely need a new gauge.
What is the serial number off your motor, please?
For starters, you may want to disconnect the tach, or replace it. You cannot test a tachometer except to replace it with a known good one, If the good one works hooked up to the same wires as the suspect tach, it's bad. If the known good one doesn't work, the problem is the tach wiring.
You should still check the voltage at the battery. If you try to do a repair and assume something works it will bite you one the butt. Not to mentions waist a bunch of time.
If your voltage is indeed low, with a new regulator on the motor that pretty much means the charge coil on the stator is bad. So instead of just replacing the stator, it's easier to check the Battery voltage ( and a lot cheaper) than it is to replace the stator
You may even have a problem with the tach shorting the power wire in it to ground. This too will cause a lowered reading on the volt meter