Is your leak at the top timing cover or the bottom timing cover? The top timing cover is no big deal as long as you use a "slick before cured" sealant on the bottom useal so that the cover while slide rearward into place without disturbing the useal.
The M103 has a ratcheting tensioner and is probably the most durable timing chain system on any of the MB's. However, the chain is cheap and since you will have the cover off for access, it wouldn't be a problem to roll in a new chain. If you do, remove the tensioner mechanism, it will require some odd size Allens. Once removed, push the plunger all the way out of the ratcheting mechanism, and reinsert from the back. If you do not take this tension off, you will break something.