Lazer Z only starts with assist of battery charger

The engine should not need such high amps for it to start. Sorry to state the obvious. From this point, The correct stock battery should provide enough amps. Tracking down why will lead to a solution. I was thinking a bad relay since a relay requires very low amperage to trigger and from there, high amps flow to the end device, whether it's a starter motor or some other high amp draw device. It's not unheard that there was a mistake made by an hourly paid employee doing the assembly. I experienced this on a Honda walk-behind mower a few years ago. If it was me, I'd get the factory repair manual and check that all wiring is correct according to the schematic.

Did you ever replace the starter relay and starter?

I still have money on those being the fix. Damned if i would screw around with a charger when this can easily be fixed with a starter replacement.