I don't think it's availble as a separate item. When the one on my KM454 quit, the selling dealer sent me a whole new cluster gauge assembly under warranty. Everything else on the OE cluster gauge worked fine except the hour meter, but he sent a whole new cluster assembly anyway.
bjr, it's easy to pick up a Hobbs hour meter and install it, about $ 28.00 unless your tractor is under warranty get your dealer to send you a replacement tach.I have them if you don't find one