She just won't pull

Hi, I'm a first timer so please be gentle, I have an 05 aerox which apart from the gianelli exhaust I believe to be standard. 12 mm carb with 86 main. A week ago she stopped running. She would start but stall as soon as the throttle was opened. I cleaned the jets and she was good. Next day it happened again but this time when I cleaned the jets there was no improvement. Some "expert" mates came over & before I knew what was going on we had stripped the barrel off. I've now rebuilt her, with a new piston and cleaned the fuel tank & carb. She starts, albeit reluctently, won't tick over and is very weak through the whole range, topping out at 15 -20mph. Where should I look first to get her sorted out?

check the oil seals on the crankshaft
Sometimes they pop out just a little, the engine runs for a little and then shuts down, of just completly shuts down and won't even start.
Even check the gaskets on the cilinder.
Check that thing where u mount the carburattor on ( don't know the name in english so fast ) little cuts or something what can couse to false air.

When mounting the new cilinder, did u cleaned the engine good where gaskets get on?
Make sure thats always clean and all parts of old gasked are removed
This can couse even problems with false air

Goodluck mate

'She just won't pull, I'm a first timer so be gentle'

Excuse me what?

What I mean is, my ped has very little power and this the first forum I've contributed to so please be tolerant. Do you have any suggestions of where I could start in an attempt to correct her performance?