Speedo and odo both are derived from the same sensor. On the K3 it a magnet assembly (2 parts) that mount on the right side of the front wheel. It's usually the little rotating magnet part that breaks. On the SPEEDO page of the fiche, part no 21 is the rotating magnet and part no 19 is the entire sensor assy.
If you haven't done anything with the front wheel lately, I'd check the connector first. I tended to break the little magnet piece when I messed with the front tire of my old C50 which had a similar arrangement.
Yes those rotors fail. The grease in them hardens then they break. Many folks have been able to glue them back together and reinstall them. Mine broke at about 7000 miles. I glued it back together and it lasted untill the bike had over 35,000 miles on it. I did take it out and regrease it about evry 7500 miles.
My GF's 400K6's speedo sensor broke a few days ago. I repaired it using the excellent body putty that I'm using to repair my 650's tupperware. It's styrene-based stuff and has additives to keep it flexible (it's designed for tupperware and for automotive bumpers etc.). Since the rotor is plastic, I like the solvent-based body putty instead of epoxy because it should give a chemical bond with the plastic. It also cures rapidly. We'll see how long it lasts.
That's a very bad place for grease IMHO. It traps all the brake dust and road dirt, which was obviously why the grease failed. There was a little fresh grease hidden in the recess of the housing which was still in good shape. Anyway, the wheel bearings are sealed so I see no reason to have grease in there at all. I cleaned all the parts with solvents and then lubricated them with some high-quality dry silicone spray which I use on my boat. I'll remove the wheel at oil changes and check the sensor and give it another shot of silicone if necessary. If it looks good after a few cycles, I'll just leave it alone.
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