On more modern equipment theres usually a strip very much like you find in a tv remote , its rubber with preasure pads ,I did know of a music shop back in the day that could order the button strips, long since shut their doors now .
I did own a Farfisa at one point , came to me in a lot which included a guitar amp .
The switching contacts comprised a gold wire running the lenght of the keyboard and another contact in each key , some of these were worn and had only partial or intermitant contact , I used a special silver loaded paint to build up the electrical contacts on the keys that were faulty ,worked great for years after. The silver paint I got from Maplin ,it was designed for pcb repairs ,came in a small glass bottle , cost about a fiver.