There must be a simple fix for these seats and needles that leak , deburr and/ or cut a deeper seat , possibly just change the seat to a rubber looking seat ,from memory the Tecumseh has a small seat that just pushes in , can't be that difficult to make something work even if you machine something up on a jewelers lathe , I've had tons of carbs with brass seats and brass needles that worked fine and no problem with brass seats and a small ball bearing to cut the fuel off.
All the problems you've had ,I would have pushed in an old brass seat from a broken primer cap.
