Might be stating the bleeding obvious, but have you tried a new (or known good used) plug ?
I have had "duff" plugs that appear fine, spark fine against the outside of the head but won't actually start or run the bike. Put in another plug & away they go.
Also had issues with duff NGK resistor plug caps giving similar issues. Check the resistance through the cap - the number in the part number (usually something like LB05F) is the resistance in kilo-ohms. Duffers are usually open circuit - they will spark outside the head, but not under compression. You can eliminate this by connecting the HT lead direct to the plug as a temporaray measure (paper clip or similar secured under the screw-on cap of the plug pushed up the HT lead end).
Happy to pop a new MZ spec. Denso plug in the post for you to try if you wish ?