That is awesome!
Love seeing these kind of projects.
This is what home building is all about.
Reminds me a lot of my earlier projects.
As you're running such big wheels you could probably reduce the gearing a bit further to 8:1 or even more.
I've tried putting RC clutch springs on these sort of motors and a 6000RPM spring was way too stiff.
It only JUST engaged and the car crawled along at full revs.
If you lighten the shoes make sure you take the exact same amount off.
Drilling holes of the same size would be the best way.
Also I would like to see more photos of how you've mounted the engine.
Once it drives there's a lot of stress on the mounts and you don't want it to come loose....