I think that there is a Yamaha triples forum though... try xs750.com.
Also - back in the late 1970s I traded my '75 XS650B in on a 1979 Suzuki GS850G (four cylinder) - which had good ol' fashioned Kettering breaker points. After a few months I bought a Martec electronic ignition system for it (I think it cost me about $85 USD) and I am pretty sure that Martec also made a system for the Yamaha triples - you might try searching the web for one of those.
My Martec work flawlessly for more than a decade till I (foolishly) sold the bike.
Pete