^^Not likely^^ Reeds DO fail from time to time, but when this happens they either break, wear out and cease to seal properly, or stick shut. If the engine was just rebuilt, I'm sure you at least looked at the reeds. Right? As long as the reeds sit flat on the cage with no cracks or gaps and can flex open, the engine will run.
Your Blasty has an air leak. Somewhere. Air leaks lean the fuel:air mixture coming from the carbuetor right out. An air leak makes the engine idle high and erratically, and it will likely die out when you get on the throttle. What did you do about the stripped-out carburetor cap? The carburetor cap MUST seal FULLY for the engine to run properly. There should be an 'O' ring on the bottom of the carburetor cap. Is it still there and, if so, not pinched or dislocated?
Most of the time, stripped-out carb. cap threads cannot be fixed. You will probably need another carburetor if the cap won't tighten down firmly onto the carb.
If you were able to get a secure fit on the carb. cap, check your whole intake tract for loose clamps, holes in the carb. boot, and missing/damaged gaskets. (Only one gasket is used on the stock reed cage; it fits right between the reed cage and the jug; the intake manifold is [supposed to be] self-sealing against the reed cage.) Hope this info helps. I:I If all is A-OK in the intake tract, it is time for a crankcase leakdown test...