Does the clutch activating arm have some free play with the cable disconnected? If not you may have inadvertently put the clutch pusher rod 180o out which means the relief in the rod for the activating arm is in the wrong place. Just a thought
If you dismantled the plates completely they may be still dry between the plates (they need oil I believe to bite) also the plate order is critical.
One of my Trials bikes a Cota 350 would sometimes feel soft (slip) when kick starting (driving the clutch in reverse) but never ever slipped with the engine running
never did find the cause.
To test a Motoplat spark strength remove the ground electrode from a good spark plug and then see if the spark will jump from the centre electrode to the earth (with the plug grounded naturally) you may need to test in darkness to see the spark. The spark should jump that gap if the system is healthy. I assume you ensured that all paint etc was off at least one frame engine mount and coresponding engine mounting point and also off the coil mount. I know everyone knows that but had to ask. I normally run an earth wire directly from the stator base to the coil mounting bolt (ensures a solid earth). Motoplats like most early electronic ignitions have little energy at kicking speed. Hope that helps.
Greg