Most engines of that time period, did not have a muffler tap on them. You had to drill and tap a hole and put one in yourself. Heck they still sell some new engines today without a muffler vent tap on them.
Generally everyone ran a muffler tap as soon as the first guy showed up at the flying field with one and tnhey saw how well it worked. It sure made setting the needle valve less tricky.

You always have a vent on the fuel tank. You would just put on a short piece of tubing or bend the vent tube so that it's tip was up above the fuel tank top. if you had it in the prop slipstream, you would cut a angle in the vent tip angled forward to get a little pressure off the air slipstream. One company used to sell cute little plastic air scoops that one could use too.

Adjusting the engine's needle meant the engine without muffler pressure means you tended to run a little more rich when the tank was full and maybe a little bit lean when near empty. But when half full you were usually right on for the needle setting. You could almost tell when you were getting low in fuel by how well the engine was running then. of course fuel tank position was more critical, but back then the airplanes had larger noses on them, so you could mount the fuel tank higher inside.

I am embarrased to say, but I have four of those K&B 20 engines myself. They are leftover from a ill fated B17 I was trying to build. The plane got crushed in the storage shed one day by some stuff that fell on it. I have been thinking about using them on something ever since then.