[HECnet] Replacement for tachometer sensors on RA8x drives?

I've discovered that three of my four RA8x drives have failed due to bad tachometer optical sensors. Apparently the material these guys were potted with decades ago gradually turns opaque and, being as they're "optical", that's Really Bad. Has anybody else had this issue? Anybody found a replacement for them? It's just an IR LED and a phototransistor in a fancy plastic housing, and that's still pretty common devices these days. I just need one that's mechanically compatible with the RA8x HDA. A modern production replacement seems like a better plan than a new old stock replacement since any old production ones are just as likely to be bad. Bob