I have a 24 hp intex b&s that smokes 300hrs on engine replaced breather and filter replaced both head gaskets cly look good ?

Check the vent tube coming from block to breather box , did you check oil return holes in heads ?
Luck

I put new tube on with breather valve had head off and cleaned out good

Check for a oil leak , check bottom of dipstick tube .

No oil leak smokes out muffler

I understand , but if there's a oil leak it can suck air in and push oil through piston rings,

No oil leak motor is free of oil leaks out side of motor is clean as a pin