its easy sort off.
2 way of doing it
first take off everything connected to the gauge.
1) take a small flat head screw driver and slowly pry with a twisting motion the back of the pressed metal band that holds the front and the rear of the case together. Pros- if done well it will look clean front and back
2) take a small small jewelers saw and cut the little metal band. Pros- faster and if done correctly much cleaner when put back together
-optional- after splitting the case clean and re lube the gears might as well since your inside of it. i used a lube that comes in a pen style container to re lube computer case fans.
next step is to go into your kitchen and grab a fork. taking the fork slid it under the tach pointer arrow thing with the middle spindle thing between the 2 center teeth. use it like a pry bar and gently rock the tach pointer thing off.
then there is only 2 screws that hold on the tach face.
there are plenty of gauges faces
http://www.ebay.com/itm/Honda-CB550-CB550f-CB500t-CB-Cafe-Racer-Gauge-Face-Plates-Decal-Overlay-Applique-/310710746600^ for example
also while your tach arrow thing is off take some model paint and paint it crispy white again with whatever color tip you want.
install in the reverse order.
1) re crimp the end with the flattest pair of pliers you got with a bunch of rubber bands over the teeth of the pliers and some cloth or whatever to protect it when you crimp it back
2) clean the cut edges very well and pre tin the edges with solder. then squeeze them together and drop another bead of solder to seal the deal. OR put a small drop of epoxy (sand-able so you can make it super clean making the epoxy flush carefully sanding it; if you get paint-able epoxy and dont mind changing the color of the ring to like black or something it will be invisible) maybe even blue lock tight then clamping the ring together (large hose clamp or rubber band). or a quick little spot weld. obviously this all takes place on the bottom of the gauge.
3) ditch all that jazz and buy this
http://jokermachine.com/products/cafe-racer/honda-cb750-parts/cb750-speedo-tach-clamp-rings.html4) or maybe one of these old timers can chime in and tell me what im about to explain. i have seen a flange style clamp ring that is specifically made to replace that ring that has a very small screw at the bottom to clamp them back tight. i dont even know what to search in google to even pull up the correct thing. i know there out there i have seen it!! this option would be hella cheap but i dont even know the name of the item to search for it