What to check is simple. You need first to verify fuel pressure and then you need to verify secondary voltage requirements. Some of this is a little tricky even with a scope.
I pull a plug wire and see how high the voltage goes (measured between the coil and the dist center term). This tells me available kv (AT THE PLUG). Unfortunately as the voltage goes from the standard 10kv to over 25kv two possible problems can look the same. If the coil can only do 13kv then thats what the scope shows. If the rotor becaomes conductive to ground at 13kv it will look the same. On older models we then would pull the coil wire from the center electrode (slowly - creating an air gap) and view the voltage if it stops at 13kv then the coil is at fault; if it goes to 25+kv then we know its going to ground inside the distributor. In the early 104 as with the 103 the dist is in front of the head with the fan in the way of this last technique.
Bosch Master, ASE Master, L1
33 years MB technician