Good luck to you! I have been trying to figure that out for about a year :?.
The only route I have come up with is to buy an entire left side control unit and scavenge the button. As a temporary measure I took the rubber "buttons" out of an old cell phone, trimmed down one of the larger button and glued it into the control unit. That at least give me a raised surface to press to activate my horn. HTH
Several years ago when I had an 84 I found an old radio with some rectangle buttons that were about the same size so I pulled one out and used my Dremel tool to modify it a little and glued it in place and it worked and looke great.
I looked all over for a small switch I could use for a starter. I found micro switches at Radio Shack. I did find them to fit perfectly. Fit and function are not the same. I couldn't turn the headlight off If connected the power full time. If not connected to power, headlight wouldn't work at all.
By using one of the micro switches for horn should be acceptable. They look great on the stock brackets and come in a 4 pack (two red buttons and two black buttons). The down side is that they are extremely heat sensitive. No matter how I tried, I found it quite impossible to solder the wires to the contacts. I believe somewhere I saw some micro blade connectors that could be plugged onto the contacts.
This is not a solution but rather something else that might be usable. They didn't work for me, I kept melting them. I just gave up and purchased a stock used set of switches.