Generally if you hear a "POP" it is one of 2 things. Either the engine timing that is synchronized to the spark has fired at the wrong time and the fuel has ignited at the wrong time or something broke. Sometimes if something broke internally there will also be a heck of a lot of clatter afterward but not always.
As I had said in an earlier post your problem sounded like a lean condition. If you run an engine in a lean condition for too long of a time it WILL, not might or maybe cause serious damage if not corrected immediately. Some damage is manageable and easily repairable and some is not.
My best guess is that from the description of the engine running then losing power and now being tight and popping after what I suspect as being a lean condition you have probably gaulded the piston and it became so tight in jug that it could no longer freely move, then stuck and the ambient temperature of the cylinder and head ignited the fuel causing the popping noise you heard. This is just a guess but one I have seen numerous times in the past.
Remove the spark plug and pull off the muffler and then try to rotate the the engine manually by the crankshaft. If it is still tight then look into the muffler port and observe the condition of the piston and ring. You will readily see if there is evidence of gaulding or scoring on the exhaust side better than the intake side for the fact that is where the hottest gasses are exhausted. An engine that is damaged from a lean burn shows the greatest degree of damage on that side of the piston because burning predominately air is much hotter than a proper mix of air and fuel which is much cooler.
If the piston is moving freely and there is no scoring connect a compression gauge to ascertain proper compression for a 2 cycle (generally in excess of 100 PSI). Then disassemble the magneto stator and rotor and make sure there is not a key that has sheared on the rotor placing the timing of the spark out of synchronization with the piston.
Good Luck.
