I would look at the engine and undercarriage first. Check oil pressure carefully, as engine bearings are nearly non-existant until someone finds an inventory someplace. Tracks and rollers need to be in good shape, as they are getting hard to find, but, if you are a decent machinist, you can make most of these parts. Still yet, the material needed is a little pricey.
And you will need to have heat treating services available, if you want those parts you make to last.
Take care!!
Andy