NOBODY can wave a magic wand and divine what the heck is wrong with your ped from over the net...especially when you don't help much.
Anybody you take it to to get it fixed will go through a process of elimination based on what you tell them.. and their experience.
You HAVE to pull the main jet out and look through it.
You HAVE to check the plug and report back on what you find.
You HAVE to check ignition timing.
So far you are not even sure if you have an electronic ignition or points.
Pull the dang ignition cover and LOOK at it !!... then tell us!
If it has points they prob just need to be cleaned and set.
And... "why would the ignition be bad now when it ran fine 3 months ago"?
Because ...SHIT HAPPENS !.... Just because I didn't have the Flu 6 months ago doesn't mean I won't have the Flu tomorrow !!
The "piston is too small for the bore" is most likely not true ... it was a new piston recently..right?...Then it is not the piston and bore UNLESS it had a seizure (or the bore guy was a total hack).. you said you had the piston and cyl off to look for what might be wrong... did the piston have ugly scuffs and scrapes on it's skirt ?... if it didn't .. then the piston and bore should be fine.
However.. I did see you said where it doesn't have an air filter.. and if you ride where there is much dirt your piston and ring life will be drastically shortened.
If I had your ped in my hands.. I would figure out what is wrong within 1 hour...(but I have all the tools too)... they are NOT mysteries.
When you asked before .. I told you twice. it wasn't the pipe.