KZ200 -80 ignition timing

Hi
I have a KZ200 -80 which I have milled(?) of the cylinder head by 1mm (sorry, english isn't my native language).
Now I think the timing for ignition is wrong at higher revs.
In the service book they refer to use of  a "timing inspection cover" which seems to be hard to find.
Any tips on how to set/check the ingition timing without that?
Or is the cover only to prevent the engine from splashing oil?