Sounds like it's time to cut your losses.
As mentioned, a valve can be bent in a manner that is imperceptible to an untrained eye, but will nonetheless cause a sticking valve or poor contact with the seat, resulting in a loss of compression and/or a host of other troubles. You may have damage you're just not seeing.
I would send the head out to a good reputable machine shop, preferably one that has M Benz experience and have it looked over professionally. Besides, you can't just put a new valve in as a replacement. It will need to be seated/checked with a valve lapper and seating compound. The new valves will also need new seals, might as well replace them all (they're cheap) while the head's off, and it's very likely that the head is due for new guides (and possibly springs). A shop will remove all the valves and check them as well as the guides, seats, springs and a host of other inclusive and important things, making sure it's all in spec. Ask them to make recommendations and QUOTE costs before proceeding. It would probably cost in the neighborhood of 375-450 +/- including parts. A small investment considering you'd be spending 100 or so plus all your time trying a short term patch-up job with any further mistakes costing 2-4 times the machine shop (peace of mind) price.

Edited to add: There are other damage possibilities as well that are often over looked. Consider the cam pushing down on a valve while a piston is trying to push it up. This would cause much stress on the cam bearing tower bolts that hold the cam in place. The threads in the head on those tower bolts in an M103 are known to be a weak point and damage easily. A cam lobe is at risk in such a situation too. More reasons to let a good shop look it over.