I recently replaced the belts on my Sears Celebrity Power 12 (a Smith Corona rebrand if I understand correctly) that were shedding perished rubber with Gates 3M315 Polyflex V-Belts. The typewriter is much happier now; no more bouncing middle wheel or rattling and the impressions are much clearer. Before the smaller punctuation characters (-_.,) had about a 75% success rate, now they work great.

I was able to buy replacement toothed belts from an industrial seller via eBay by measurement. Actual Gates belts. Mine fit a Smith Corona electric 6TE and the part number is 3M315 but once you know the width and length it’s a matter of translating it into a Gates part number.