If you are going to replace the speedometer anyway, just look around for one that has a trip meter and adapt it to your bike. In the mid 80's the big four standardized the ratios so they will interchange from that point forward. Several of the Suzuki speedometers are an easy swap. The handlebar mounted ones will work if you use a longer cable (tach cable for the old GT750 water buffalo works nicely). Or you can do an entire front end swap from a smaller Intruder (a bolt on swap) that drives the speedometer from the front wheel. Many options out there if you look around.

Name or link? Might be helpful

The people that give the negative reviews for these speedometers usually don't know how to install them or understand Electronics because the instructions are extremely vague