If you put a spoonful of fuel into the cylinder and it wouldn't fire, the problem is not intake sealing or carburetor. The first problem to consider is whether the spark plug is fouled. You said it has spark - did you check this with the actual spark plug you intend to use? Was the spark blue and strong, or red and weak?
If you are sure it has good spark, it sounds as if the compression may not be as good as you think. You could try removing the decompressor from the cylinder head and temporarily screwing in a second spark plug there to plug the hole you took the decompressor out from. Put a screw into the air hose that goes to the decompressor, to plug it. Then try the compression again. It should be extremely strong, so it is quite difficult to pull the starter cord. If it isn't all that strong, there is one further test to confirm the problem: leave the plug that replaced the decompressor in place, take out the original spark plug, put one spoonful of clean oil (engine oil or 2 stroke oil, it makes no difference) in through the spark plug hole, then put the spark plug back in but do not connect the wire to it. Pull the starter again, and see if it has become harder to pull. If it has, you have a compression problem.
