gg250f loss of power question

[forgiven]

OK here is the deal

I have a 2011 GasGas 250f which is an enduro with a 2006 wr250f motor. It doesn't have a grey wire or the other restriction stuff. I bought it used and seemed to be low on power. I put an 06yz250f piston in and no change, and the old one was fine, put in yz exhaust cam very little change. Megabomb header and 4.1 slipon and NOTHING like my other bikes.

I have a 2006 yz250f with a flywheel weight which is a beast. I have a 2013 yz250f which is also a beast. Both are stock minus an exhaust on the 2006 but the 2013 is completely stock.

Any ideas where to look. It does ok on level ground but under a load like a hill it shows its lack of power. Air filter is clean, oil is new. I am at an absolute loss. IF I could get electric start and power similiar to my 06 yzf this might be the best bike ever made by man so please help.