This is a comment about the "should be wired with a positive ground" statement; I don"t think it is good to get that embedded in the thread without contesting it. John, I know you have a world of experience and knowledge about these Fergies, but most autos for decades ran with negative ground. I run my TO-20 quite happily with a negative ground. There are things that must be done to switch to a negative ground system but they are very simple and it works perfectly well. Note that DIS (waste spark) automotive systems run with one plug getting positive spark polarity and one plug getting negative. The difference is where the most erosion of the plug electrodes will occur, and on a +ve HV plug the center electrode will tend to have more erosion because the +ve ions have more mass than the -ve electrons; but the center electrodes are typically made with more durable materials than the grounded protruding electrode. And I don"t really think most of our tractor plugs die from electrode erosion anyway. But this could be a discussion for another thread!
My advice for poor starts:
Good heavy battery cables with very clean connections at all points (terminals, frame ground and at all points to starter. Plus clean starter to frame mating surfaces.
And check your starter - check the brushes and commutator surface; shouldn't be too much scoring and the brushes should float freely and be long enough and show evidence of mating all across the commutator and not just in a small strip (can judge but color and wear. Make sure the commutator segments are not shorted.
And make sure you have changed gas with the season - the winter blends are more volatile and if you have old gas it is likely going stale anyway.
Storing in a shed is the best most of us can do but tractors are massive and condensation and moisture will work to corrode and oxidize things like electrical connections. Store inside if possible; regular use is the best medicine.
And don't mess with too many things all at once! If it was running in summer, it is probably pretty close to being right! These tractors with their 6V systems worked reliably for decades and there is no good reason that can't still be true; but time allows all the degradations of electrical contacts and connections and that is the most likely issue.
Best of luck with it.