Recommendations for a good audiophile digital coax (BNC-BNC) 50 ohm cable?

Oh, I see the Mutec has both 75-ohm and 50-ohm outputs; I’d guess the K1 has a 50-ohm circuit, so Mutec suggests you match it.

If you bought one of these Amphenol cables (almost free) you would at least have a baseline for assessing the boutique ones. Amphenol has been designing and selling radio-frequency cable and connectors for generations for commercial and military use. It’s hard to imagine that a specialist cable manufacturer working out of a small shop knows RF technology better. With such a cheap baseline, you can decide for yourself (you might want to ask Mutec their opinion -- they are sensible people).

Most wordclock connections I’ve seen specify a 75-ohm cable. I don’t know if that’s the case for your gear, but 50-ohm connections typically are used for circuits carrying considerable power. May be worth a check to make sure you get the right cable for the job.

I sincerely doubt you’ll have anything to complain about with a standard cable assembly made by Amphenol or other such vendor, but of course the boutique cable manufacturers do make a much nicer looking product.