The Motorcycle's electrical system isn't as noisy as a car's and doesn't have all the current / voltage spikes as in a car.
Got any facts to back up this statement? If anything, I'd expect that a motorcycle electrical system to be noisier than an automobile's, since most motorcycles don't have radios and other things that require quiet power to function well.
The motorcycle generator's voltage varies with RPM's and also with the current battery state - ranges from say around 10 - 14.5 vDC.
Most car systems have tighter tolerances - more like 12 to 14.5.
With a decent heat-sink, creating a stable 5V source using a 7805 regulator shouldn't be too difficult.
9. I am told to use a series resistor to limit current after an in-line 1amp fuse - what value and power rating fpr the resistor?
That depends on what voltage drop you are trying to achieve, and how much current is behind that voltage.
I also read I might need to provide a separate voltage regulator to step down from 12 vDC to a lower voltage to avoid overheating the arduino's power linear regulator?
Certainly a good idea.
14. For the 12vDC monitor, the gear position, and the engine coolant temp monitor, I am planning to use just a simple resistor voltage divider to rescale the analog voltages between 0-5vDC?
Is this a question or a statement?