Had all the same symptoms last week wiith my '89 Derbi of many years. Here's what I did.. Now she runs like new and starts right up! Removed and cleaned the fuel tank. Removed and replaced the petcock (filter on it sits inside the tank and was pretty gunked and there was corrosion inside...probably could have used the same one after taking it apart and cleaning it, however) removed, took apart and cleaned the carb (loaded with gunk). Replaced the in-line fuel filter (be sure it's set in the right direction...not all replacements are marked as such) I use the clear view type with the white trans screen i find best for 2cycles...and you can see) If the flow direction isn't marked, can't hurt trying it the other way).
Before all this it was running... Then just stopped..having all your exact symptoms. Like you, Starting fluid only allowed me to know there was a good spark... So I just worked back from there covering all bases with fuel flow. Like Richard S says..spark + fuel = combustion. Let me know if I can help with any questions. Best of luck.