Gonzo I wouldn't worry too much about the torn area unless it's really bad or you bounce around on it. Either way if you don't nourish and hydrate it, it will eventually fall apart.
Gonzo try using Neatsfoot Compound oil by Carr, Day and Martin it is quick acting and deep penetrating. Apply it not too liberally on the underside and leave it to soak in overnight. This will nourish and re-hydrate the leather. I swear by it and it has never failed me.
I'm looking to buy a new Wheel Jig, one that I can both build and true with. Can anyone advise which type to get.