I'm Still Standing

I got 650SX '87 about 2 months ago that fired up fine, idled fine, ran hard fine for a full tank. after i put in the second tank of gas at 1/2 tank it Started missing and seems to be putting out quite a bit of white smoke at the exhaust. Details:
1. Was running fine through an entire tank of gas before being refilled and 1/2 way through the second tank started giving trouble
2. I am putting premix 40 to 1 along with Stabil fuel stabilizer.
3. Plugs relatively new (couple tanks of gas) but anyways cleaned plugs and fired her up but still same problem
4. decent stream of pretty warm water coming out of pisser
5. engine misses both under load or on the stand
6. when I pulled the plugs the font seemed a bit wet
7. both plugs seem a bit dark in color
8. Compression is 120 in front and 100 in back....may need to re run test
9. problem is happening with engine cover on and engine cover off

Thinking I need new head gasket but before I pull this thing apart (new to the jet ski game) I am asking for some feedback I dont know crap really. I live in Miami, FL and want to get out on the water while its still warm. How bout it??