Dear Townshend Act,
Hope you're well and stay so. You have a plumbing problem, so use what plumbers use, when joining sections of threaded pipe, i.e., for gas, steam, or water.
Trying these materials - pipe joint compound (gray, not white) or white teflon tape - will cost only a dollar or so each. If one of these materials stops the leak, you've saved a pile of money versus pulling the head. If they don't work, you're about $3 out-of-pocket and still have usable pipe-sealing materials.
As the surfaces involved likely have on them a film of oil - that will be next-to-impossible to remove - try the gray, oil-based pipe joint compound, that plumbers use when installing black iron gas pipe. See: https://www.acehardware.com/departments/plumbing/solvents-and-cements/plumbers-putty-and-sealers/4386280 . This pipe joint compound is rated up to 400°F. I doubt the oil gets much above 200°F.
Do not use white, teflon-based pipe joint compound. The gray pipe-joint compound is required for use on gas pipes as any particles of the joint compound are so fine they won't plug orifices on gas appliances. That is not so of the white pipe joint compound. You do not want any large particles entering the engine's oil supply.
If the allen-head plug can be tightened completely - if it does not "wobble" as it is being tightened - the gray pipe joint compound's high viscosity should create a lasting seal. This pipe joint compound is oil-based: an oil film on the surface of the head's oil gallery opening and the plug's thread should not be a problem. That's not so as to the other materials you tried: they need surgically-clean surfaces to bond effectively.
In any event, it will cost only a couple of dollars for a tube of the pipe joint compound.
After installing the plug, I'd warm the area, using a heat gun and then let the plug set for a day or two, before starting the engine.
If that doesn't work, try using white teflon tape, that plumbers use on threaded joints for water or steam pipes. If the threading on the allen-head plug is slightly off-spec - such that the plug's threads do not fit the opening as tightly as they should - the tape's thickness will "bridge" any gaps. Here, too, a roll of such tape is about a dollar: see: https://www.homedepot.com/p/1-2-in-x-520-in-Thread-Seal-Tape-31273/202206819 .
Teflon tape also does not require surgically-clean surfaces to work correctly, so long as the fitting can be tightened. If the allen-head plug is a loose-fit, the tape should still work. You may need a couple of tries to figure out how many layers of tape to put on the plug, so that you get a tight, lasting seal.
Hope this helps.
Stay well!!
Yours faithfully,
Spook
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