It depends on your problem. The issue could just be that your speedometer cable, which runs from the back of the speedo to the transfercase in my 78, is broken. If you search around you can find testing procedures to determine if this is the problem. Or the problem could be with the actual speedometer magnets which move the needle when your cable spins. This case would require that you replace your speedometer.
In response to your other question, the speedometer assembly is held onto the dash on the back side with 4 3/8" nuts on my 78. Extremely easy to take off compared to some things on the Jeep...
With aftermarket replacements I've really only seen the ones that go from 0-90 mph, 5-whatever mph, and the ones for km/h.
1978 CJ5 - 304 V8 - Twin Sticked and sitting on 35's
2010 JK- Rock Krawler 2.5" on 35" KM2's