update 6
gnah! i think i reached a dead end here.
so the mounting plate now is holding up fine. but the plate is so thick, that the small pulley only is inserted 1/2 of its size which harms the pulley quite alot. also, i took a few extra bumby roads and the clamp holding onto the truck now broke. i cant make the clamp thicker but i could make it wider like SimosMCmuffin did. i was thinking about that before, but decided aggainst it.
never the less. I think i stop here and just buy a metal mount instead. the motor is just to darn heavy and to fix this properly i would need to mod the motor itself which is out of my abilities.
I think you could improve on the geometry to avoid cracks. Use large radiuses where you can, instead of sharp corners and circular holes, not slots. Preferably also without chamfers/countersunk.