To start with, I think an LED lamp of some sort will be the most power efficient so I would try to find one that is a cluster of LEDs about an inch across. Suppliers like Digikey would be my first place to start on that. One nice thing about LEDs is that they respond so quickly that if you get hard up for power, you can pulse them on and off quickly enough so that the eye doesn't notice the pulsing, but enough to save some power, since its off part of the time.
As for the electronics, the first thing to come to mind is the LM2917 which can be used as an electronic tachometer. This would be an analog solution, so the precision of the shift point may be ok or may be not good enough. It is also going to be a bit of a pain in the neck that it wants a higher voltage than 3V to operate and draws more current than I think you can afford. So, a digital solution might be better. For a digital tach, which would be more precise, any AVR or PIC with a counter-timer function could be set up to work well. An AVR, like for example the ATTINY2313 series, can operate at 2.7 volts and draws very little current.
The user input to the AVR or PIC can be some sort of decade switches ($$) or some up/down buttons and a simple display which would be cheaper.
To pick up the spark, most tachometers tap into a low voltage input to the coil, not the high voltage spark lead. But if you must tap the spark lead, do it with a capacitive or inductive pick-up, not a direct connection. Avoid a direct connection to the spark lead as the high voltage is difficult to control and use safely.