i've had fair-to middle success with a product called PLASTEX, sold online and by phone by GT MotorSports. powder and liquid mixed. repaired chipped keys, broken cases, even reproduced slider knobs. messy, expensive, and 'fiddle-y' but holds well. only comes in limited colors - black,white and clear, so you'll have to match paint. in the kit is a rubbery bar that - when heated- can be moulded to another tab. when cool it is firm and makes a mold. fit the broken tab in place and fill the void. mold can be re-used for the other broken tabs. Also the same tabs were used on some Multivox synths (mx-2000, for one). good luck
"A thing may seem impossible...until it's done."