Busted rockers... we've all had 'em. After all, it's 40-50 yr old plastic we're dealin' with here. Here's one way to fix 'em if you don't have access to a 3D printer.
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So first off... cover your tank. Don't want a screwdriver slippin' and scratching the paint... do we. 2 screws on the bottom of the switch and you can peel the two halves away from the handlebars. Inside you'll see a little bracket holding the rocker in place with 1 screw. Remove it.
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And now the rocker will pop out of the housing..
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Clean the rocker with your solvent of choice to remove all dirt and grease, then rough up the broken end. Easy with the solvent... lest you melt the part. It's kinda small and fiddly for a file or sandpaper so I just used an Exacto knife and scraped across it. Roughed it up nicely.
Next I used some aluminum tape to create a mold to duplicate the original part. I made it slightly large so that I could use some sandpaper to finish it down to size.
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Then I poured in some J B Weld original formula. After you pour it, use a tooth pick to stir it around. That gets rid of any air bubbles and makes sure it's well bonded to the plastic. Let it sit up overnight to fully cure.
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Peel the tape off and you should have the rough shape of the tab.
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Using sandpaper on a block, slowly work it down to size... checking the fit frequently with the button.
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The original Yamaha screw is self tapping, which I feel contributes to the weakness of the tab. I decided to drill and tap it for a 3X.5mm machine screw.
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And here we are. Ready to reassemble.
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Put a little dielectric grease on it....
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Push it back into place, reinstall the bracket and reinstall the switch on the handlebars
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Reinstall the little brass slider plate and screw the rocker button back on.
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And here we are....
Cheaper and much more satisfying than a new Mikes XS switch.