I just learned that the elbow presses into the carburetor and if it swivels, as mine does, it is broken and must be replaced. A new one has been ordered. Thanks for your consideration.

It's very possible that your replacement will contain the steel piece that presses into the carb, along with the white nylon piece that you see is loose. It's a two piece part.

I would gently pry the worn nylon piece off the steel piece in your carb and press the new nylon (gently pried from the steel fitting) onto the existing steel piece still in your carb.

I ordered the part number shown on the parts list, 631807. In package marked as genuine Tecumseh with logo, address and all the right markings. Made in Czech Republic. But the fitting would project from my carburetor too far- looks as if it would hit the side of the engine. Tight place there. So I will use epoxy to try to secure the old plastic, which is not broken but is slightly loose, to the metal part. That is the next thing I will try.

I'm thinking the plastic part is polyethylene
and nothing sticks to that stuff, (for very
long, anyway).

The original does not look nor feel like polyethylene to me. Nor like polypropylene. Might be nylon 6/6 or one of the "cousins" of nylon. Might be acetyl, looks and feels like it. But the part was originally molded over a "knurled knob" on the metal part. The "knob" has a hole for the fuel flow. It still fit rather snugly on the "knob". I hope the epoxy will serve as a leak seal even if it does not bond to the plastic.

I've encountered those things very loose, to the point that you could pull the nylon off and put it in a new position and they wouldn't leak.

I would try using Permatex Aviation Forma -gasket. It is sticky and non hardening.