The shift drum stopper can be fixed on that 125 even if it broke off the boss... I dealt with that once years back. I took it to a local welding shop and he cut a piece of round aluminum stock and welded it in place then i drilled and tapped the center of it for the bolt and put it back together. You do need to know the height of the boss, i luckily had the one that broke off but you could measure another one if you didn't. Then just lay the stopper on there and mark the hole location to drill and tap.
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....