I'm not familiar with you particular
engine, but generally speaking, the back of the most
engine gauges will have a removable insert that carries the small 12vt d.c. bulb. A good auto
parts store should have a replacement bulb.
Again generally speaking, the glass covers of gauges are not removable and are sealed to the case when the gauge is manufactured. Moist air can enter the gauge body from the back through the light fitting or through the holes where wires enter the body (some gauges have terminal studs on the back, and some have wires coming out of the body). In any case, it would be a good idea to reseal the panel especially if it is in the
cockpit and exposed to
water.