It is difficult to say at this point, but you need to be certain that those gaskets are not leaking: it won't run properly if either of them leaks. You might try the fuel-into-the-air intake or into the spark plug hole trick again. If it runs better when you do that, than it does when you supply the fuel through the carburetor, then there is either a fuel system/carburetor fault or an intake leak. If it doesn't run when you do that, it may (emphasis, may) be the head gasket.

Usually a gasket has to leak very badly to cause it to not run at all. Sometimes when you reassemble an engine the cylinder or cylinder head jams on the studs and doesn't sit properly on the mating surface. That kind of problem can cause a complete non-start or almost non-start. A guess: perhaps your original problem was the coked muffler closed off the outward flow from the exhaust port, but your investigation has caused a problem in the gaskets, the fuel tank/fuel line, the carburetor, or the spark plug. Example: there might now be crud blocking the fuel tank filter, where it wasn't before. Or, if your fuel tank filter isn't working, a fresh supply of crud might have moved down into the carburetor needle and seat. Another example: as you said, there might now be a major leak in a gasket. A third example: when you reassembled the carburetor, you may have got something wrong (especially if it's a Victa plastic carburetor: it takes a near miracle for a new player to get one of those working after reassembling it).

Hence my suggestion that you try the fuel-in-the-plughole trick again. If it fires up decently but briefly, look at the fuel system, carburetor, and intake pipe/crankcase sealing. If it doesn't, you may have a fairly serious head gasket problem.

Don't feel badly about the possibility that you've caused a problem or two when you worked on the engine: most of us do when we first try to do these things, especially if we don't have a reliable mentor who is physically there and looking over what we do. I had the good fortune to have a father who was both experienced and helpful. When you have to learn by a boot-strap process, it is tough going at first.