Not starting up anymore!

Does the engine have spark?
Is there confirmed fuel float to the carb? How about into the float bowl?

Yes, but does it have spark?
If the plug is black, but not wet, is it getting fuel?

I wonder if you seized the piston in the cylinder? take off the exhaust pipe, shine a flashlight in the exhaust port with the piston on the bottom of its stroke see is you have a nice smooth cylinder or some nasty scratches in the cylinder. You will also be able to see the piston you can bring the piston up in the cylinder by slowly rotating the crank shaft, if the piston skirt is scratched deeply or you see what looks like nasty grooves and smeared metal then you won the seized piston award. It will then be rebuild time or replacement time.
The exhaust side of the piston is where the thing will seize most of the time.
If you have a leak anywhere on the engine where it can draw in air that will lean out the fuel mixture resulting in a lean condition and a little too much load on the engine when new can
make it over heat and seize the piston to the cylinder wall. It will seize in a small spot on the piston to cylinder wall in a spot where the metal is just a hair too tight due to crumby machining and fit of the parts.
It take about 3 to 4 tanks of fuel for these engine to wear in some will wear in quickly some take longer. These engines can poo the bad at any tme.