Hi David,
I know you've got your heart set on the ammeter, but please let me try and dissuade you.
The line from battery to instrument and back with connections and meter adds series resistance to the battery feed
The whole thing adds complexity into a critical part of the circuit where any failure will result in the vehicle "failing to proceed".
A voltmeter can tell you as much (or more) - if the battery is not being charged the voltage will be low, if the regulator fails it may be high.
If there's a short or excess current drain, then again, the voltage will be low.
If you haven't ridden it for a while and wonder why it's hard to start the voltmeter may shed some light on this matter.
If I can't convince you, can I offer an alternative? An ammeter merely measures the voltage drop across a small resistance (called a "shunt").
In the meter, the shunt is internal, but there's no reason why you couldn't get a current shunt and place it right at the battery and run low-current lines to a meter at the front. If you wanted to be clever, a simple switch and resistor arrangement could be used to put the meter in either voltage or current measurement modes. The best of both worlds without any of the pain. Shunts are readily available from electronic component suppliers such as Farnell, RS, Mouser and Digi-Key. They are available in ranges for fsd of mA to 100s of Amps.
Just my 2c worth.