Hi John,
This will do what you want but you have left out some information that I have had to make a guess at.
For C1 you can use a large electrolytic cap'. The bigger the cap' the longer the light stays on. As I don't know what light you are using I can't say how long. A super cap is better, but more expensive and they are relatively low voltage devices (usually 3.3V or 5V) If you apply more voltage then they will fail.
It works like this. The variable a.c. voltage from you generator (alternator) gets rectified into a variable voltage d.c. and this lights LED D2. When the voltage is higher, the +ve input of the comparator (LM311) is > -ve and the output is HI turning on the MOSFET Q1 and so the second LED light.
regards
Pete
