Yes that is a interesting thought as to why they didn't have a muffler pressure tap on the engine for pressurizing the fuel tank. Probably the accountants got involved in cost reduction at the time. But the carburetor is in the rear and much more close to the fuel tank though, so maybe it wasn't a problem.

Well things similar were done in the past. Fox originally made a .049 engine for COMET when Comet was going to sell some RTF control line planes. But the planes came out on the heavy side, so Fox had to up the displacement to make the engines .07's instead in order to compensate for the added weight.

I suspect that the Gilbert .07 engines for the Gilbert RTF control line planes had the same problem in that they likely upped the displacement from .049 to .07 to compensate too. The .11's were probably .09's at first for the same reason.

The .06 engines and .07 engines we see that were being sold probably had the same problems with mufflers and RC throttles, glow plugs, etc. Thus they had upped the displacement on them from .049 to compensate too. You see that with the Norvel .049 and .06 engines and the Thunder Tiger .07 engine too.