Hi,
  As Chris said, it would be worth taking the carb off and cleaning it, but also at the same time consider rebuilding it. ...not much of a job. Parts should be available through your local 
AGCO dealer, or Maibach.
  As for the manifold, I would take it off and give it a visual inspection inside for cracks and those rusted away parts that were spoken of. If you want to know if there is any exaust leakage between the intake and exaust it is easy to test. With the manifold on the bench, fill the exaust side with water. You should have no water showing/leaking through to on the intake side.  Any cracks I would weld or have welded. Just my 2 cents.
Keep us posted as to what you find.
                           Cal