I made a RPM/speedometer attached to my bicycle. I used a reed switch and 2x7 segment display. After some difficulties with interrupts it finally worked. I based my code on a code I found on the Playground and showed the rpm of a computer fan. I changed that and added the display code, voila !
It should´t be to difficult to hook up a simillar system like mine but in stead of reed switch use the spark signal. There are also some kits you could buy, I remember one from a Greek company but I can´t remember the name :