MB has a service instruction on how to tell where the leak comes from on the front of the head. Basically it says reseal the cam cover and if it still leaks then its the head.
Let me tell you from a shop owners point of view. the customer won't hear it. We no longer offer the cam reseal on 104 motors without the head gasket repair. Although every supplier claims to sell the same gasket as MB. I have checked as recently as last Christmas and only the new OE MB gasket had the perforated stainless steel inserts in the front corners.
We have no trouble making the thing seal with the proper redesigned gasket and the new sealants.
One side bar as to the chain tentioner. The tentioner is in the lower cover and the oil feed passes from the block and through the case to the tentioner. The passage is sealed at the cover with a gasket that in one direction is only about 0.33in from an attaching bolt hole. Let me tell you, that was one hard leak to find and we did it with the flourescent dye as mentioned above.
Don't try to seal a leak with goop from the inside out. I can't then fix the problem for someone who cares. If you have got to use some goop, clean the outside and goop it up there. There are actually some products now that might work. Not that I would suggest such; as by the time you have exposed the area to cleaning and treatment you could be half way done with the proper repair.
Bosch Master, ASE Master, L1
33 years MB technician