Fred,
I do believe that pretty much most if not all parts are available. Years ago when I needed parts for my D6 9U Finning, our local cat dealer, told me they can get me any part I needed. Berco would probably have all the parts you need for your undercarriage and I'm sure there are others too, not sure what the cost woud be though... I'm sure Quiktrax would know, or at least be able to give you a better answer than I ever could. My newer 977H had the tracks worn right out but the older one had a new undercarriage on it. Since the engine has a miss on the older 977 I've started swapping all the undercarriage bits and pieces over to the newer one. The older one even has segmented sprockets on it, unfortunately the new one doesn't but it's sprockets are still in great condition, otherwise I'd be very tempted to swap them over too. I'll attach a few pictures of the older machine. The first picture is after I got it running for the first time in probably 10 years, the second is unloading it off the lowbed at home, the third and fourth are my brother in law driving it around. As you can see it does smoke a little, hence I'm not using it much. Since the pictures were taken I've found a bucket for it but haven't had the time yet to install it but hope to soon...
On edit: Ok the pictures all posted out of order... Pic 1 and 4 are my brother in law driving it around, #2 is unloading it at home and #3 is the first time it ran in years...