I think the factory used 8 gauge for the battery/ground/starter cables, but that is pretty marginal in my opinion. I would use 6 for those if I were making one. Everything else can be 14 gauge.
The connectors are 'Packard 56' and can be found from a variety of sources on the internet. You will need a special crimper (~$30 or so for a cheap one) for them. If you don't already have the tools to do this, or are only going to do one tractor, it is probably not cost-effective to do it yourself, but if you have several tractors there will be some savings.
The woven wire loom is available from McMaster-Carr.