Forget about retightening the head bolts. That is not the problem and won't help. The problem comes from the fact that the head gasket ends at the end of the head and the seal that extends under the front cover is a pliable rubber. The two are originally pressed together, horizontally by the fact that the front cover seal is squwished vertically and expands horizontally. After a period the front seal becomes fixed and the head gasket receeds a bit reducing the compression seal.
The real fix requires a combination of the newer redesigned head gasket (with stainless inserts at the front corners - accept nothing else) and a secondary sealing agent of the newer technology. Permatex's "Right Stuff" is such a product.
If you are up for playing around you could try using a sealant such as this from the outside. It will have to be perfectly clean and dry and be allowed to set up properly. It will make a mess but might put off the inevitable for some time. You wouldn't catch me dead doing this but you are searching for an alternative. If you do it seal at least 4-5inches.
Bosch Master, ASE Master, L1
33 years MB technician