Im not 100% on the parts needed, but i know you CAN do it. You will need to get a YSR80 gear that goes inside the motor, YSR80 tach cable, YSR80 tach and im sure a few other tid bits.
This has been done before. But they are EXTREMELY hard to get. Iv tried sourcing them from Japan, and didn't find to much luck. You would think that the YSR would be popular in Japan... not at all. Alot of the YSR50/80 parts are obsolete in Japan. If you can find a complete set up in the US the go for ALOT of money.
I suggest going another route. Find a electronic style. I just did a custom set up for a friend. He was wanting a YSR80 tach set up too, but couldn't find any good deals, so I sourced him a used Dayton tach and made him a custom carbon fiber holder. I think it turned out ok.