It would require a lot of time and effort, but I'd suggest removing the timing cover and reshaping the Woodruff key slot with a cutoff wheel on a Dremel tool. Then a custom-fitted key could be filed for a tight fit to both the crankshaft and the balancer.

To do it right would involve removing the crankshaft, welding up the key slot, and cutting a new one with a Bridgeport mill. A replacement crank would probably cost less.
Jerry

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