Your generator is a different model/family than mine, so the comments about the location of the starter may not apply. I would still expect that theres some access panel in the green case you can see, though I'm not familiar at all with your particular model so I might be mistaken.
My generator starter uses somewhere in the general vicinity of 100A when cranking, obviously a little more for the first second when it's having to overcome the inertia of the engine not rotating at all. Measuring voltages while cranking at various places will tell you if and where the voltage may be dropping.