The short answer is you're going to need more parts than what you have, plus more knowledge.
The easiest way to tackle this would be to use a microcontroller; unfortunately, you would still need the electronics knowledge to interface it with your engine, then some programming chops to create the code.
The concept of an RPM counter, though, is fairly simple - you just have a trigger (off the magneto, the spark plug, or perhaps with a hall effect sensor) that starts a timer counting upward, which is reset every rotation; for instance, let's say the motor was turning at 1 RPM - so it takes a minute to rotate once; at TDC you start counting seconds, if you count 60 at the next TDC, then you are at 1 RPM. For higher speeds, you would obviously be counting milliseconds, and you would have to do some division in some manner to get your final display, but that's all simple math in the end.
Doing this with discrete components and ICs is certainly possible, but it would be much simpler to implement with a microcontroller in the end.