If I am not mistaken HPI engines take standard long plugs and not the short OS plug but double check. I know My HPI 4.6 does. I would think if your head were leaking that bad you would be able to hear the compression leaking or see bubbles while it was running. Yes check the exhaust for leakage. If you are running that cold you have to be running so rich that even a small leak will look bigger then it is. Clean it all off and run it for a short time and see if you can tell where it is leaking.
Good Luck!

One other thing you might want to take your pull starter off and makes sure screws in the backing plate are tight!