The gear cluster shouldn’t usually give much trouble but older XS650s can sometimes develop a fondness for jumping out of gear. Careful inspection of these dogs should reveal any problems, rounding or wear on the pins indicating a problem. I don’t normally strip gears from their shafts unless strictly necessary but if replacing a single gear cog, support the gear shaft upright in a vice and carefully remove each gear, circlip and washer before replacing in the same sequence