2006 Honda Rincon 680

Hi All. I have a 2006 Honda Rincon 680 and there is no top end power, sputters with the throttle maxed out. The coil has been replaced, the fuel filter/ strainer, injector, O2 sensor cleaned, new spark plug, air filter cleaned, good gas and it still putt putts. What else could be dirty? The throttle body may still have some gunk but since the strainer was replaced it’s been running clean gas with a dose of treatment through it as the strainer was full and just absolutely filthy. Also the bike seems to run the same with the O2 sensor in or out with a finger over the hole. How can I tell if the O2 sensor is working?

Any help with the next ten things would be appreciated. Also this forum has been of immense help over the years!