most mechanics don't know squat about mopeds.. and when you or me or whoever can't isolate a problem we tend to favor replacing parts until something works.
A stator plate is a piece of metal. The ignition and lighting coils are mounted on it as well as points, perhaps condenser.. I dunno why a new plate would be needed unless it included all new electrical components.
A progressively weaker spark does suggest a bad condenser.
You can do a quick check of a condenser with the multimeter set to "continuity" or "ohms" . After disconnecting the condenser from the circuit, connect one meter lead to the condenser's wire and one to it's metal can.. It should show zero ohms.. zero continuity.. Anything else means it's internally shorted.
But the wire or the internal plates could be broken and disconnected and you will still see zero ohms. So, to test this, charge the condenser with a single 1.5 volt flashlight battery. (Connect the wire to + on the battery. Connect the - on the battery to the can.) Charging only takes a split second. Be careful and prepare ahead of time so you don't need to touch the condenser at all during or after you are charging it up.
Next, for safety, hold the wire's metal clip in a pair of pliers with insulated handles, lay the condenser on the workbench and touch the wire to the can.. you should see a spark which tells you the condenser has a 99% chance of being OK.
Since this small condesner is only being charged to 1.5 volts, danger is minimal.. but never mess around with condensers (capacitors) because the discharge shock has been known to kill people.