Wow! I don't recall a post that ever started out with that much information. 'Grats and thanks for being so thorough.
Your problem is most likely going to be too much air or not enough gas. On the first, you do need to check for an air/vacuum leak somewhere around the manifold. It could be as simple as loose or deteriorated gaskets, either at the head or at the bottom connection to the neck of the carb. Worse cases might be a crack or hole in the manifold, or a blown head gasket (a compresssion check might be in order to eliminate the latter). The easiest test is to get it running, and then spray a little aerosol carb cleaner into the area. If the motor stumbles or bogs down further, that would indicate a leak, and a more careful control of the spray from that point will help you locate it more specifically.
It does sound as if you've worked over the fuel supply side. You could try eliminating or replacing that inline filter (be sure any new filter is compatible with a gravity feed system -- many are designed to require the pressure of a pump and make for problems on the gravity feed tractors).
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