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26inch wheels with fixed compatible hubs aren’t readily available
A Walker brother is still doing Fixxer clones to order. There must be millions of 26" MTB wheels out there with compatible Shimano freehubs.
If you don't mind losing the rear brake (or you already have rim brakes), a 135mm 6-bolt disc hub flipped over with a 6-bolt sprocket aligns reasonably well with the middle ring of the MTB triple crank found on most of these old bikes.
That’s probably it. If I only had one 26er, I’d definitely keep it geared. As I have a few though, I’m definitely tempted by the idea to try one fixed but the fact that 26inch wheels with fixed compatible hubs aren’t readily available is a bit of a hurdle and I’m not quite sure whether it’s worth the effort.