Last bike I had that had a gear indicator was a 3 speed english racer in the late 60's. I know it's slightly more common on mountain bikes, but are they really needed?
As for DIYing them, if they are Shimano, you can usually find the exploded diagram on their
tech doc site. Some of the other brands have this info too if you look hard for it.
Some shifters are inexpensive though so it might be worth getting new.