You have to find out what you have to work with, and work with it. For example, if you do not have a 12v electrical system, most common gauges are useless to you.
If you have a single-cylinder engine, you can't use a multi-cylinder tach. Further to that, even a tach from a motorcycle engine probably triggers differently, your engine may not support it.
Most temp gauges are meant to be immersed in coolant, and thread into a boss in the engine. You do not have this threaded hole. If you wanted to, you could probably just epoxy it to the cylinder head, but again- got 12 volts?
Basically, what I'm getting at here, is that except for the Tiny Tach mentioned above, and a very expensive racing unit- your gauge choices are almost none and would require some serious thought to adapt to your application- If even possible at all (but mostly not).
I (and others) have managed to make use of a bicycle speedometer. Check this thread-
http://www.diygokarts.com/vb/showthread.php?t=7450&highlight=wanna+speedo