I pull the spark plug on my 2004 Yamaha yz450f and it was full of antifreeze. What should i do next?

I think your block is cracked leaking antifreeze into your engine.. :shrug

probably just a headgasket really.

Blown headgasket or cracked head/cylinder, either way your getting a non compressible liquid into the combustion chamber.... not a good thing.

not to mention getting atleast 50% water in the oil. Also not good.

Pull that sucker apart and figure out the problem! Make sure you take pictures, too!

Sort of easy, just labor time. Good luck

Long live the 2 stroke