Welcome.
If the TORS system is still in place I would check that for proper adjustment or delete it
My daughters Blaster recently had a problem where it ran like absolute crap, no power and would barely stay running. After jetting up and then down there was no change. Turned out the expansion chamber pipe had a mouse nest in it severely restricting flow.
If the exhaust is in good working order, I would be checking for air leaks at the carb boot, reedvalve, base gasket, crank seals and maybe even the head gasket.
Is it responding to jetting changes at all?