I don't have any idea how hard it would be to adapt one to a Triumph, but the 400 - 450 suzuki twins in the late 70's and later had an LED gear indicator run by a small encoding wheel attached the end of the shift drum shaft.
If you find one and it's still on the bike, don't buy it untill you see it work. The voltage regulation on those biks is total crap and the gear display is the first thing to fry after the headlight when the volts go up.

Let's hope there's intelligent life somewhere in space 'cause it's buggar all down here. -- Monte Python