I use this extruder with i3 adapter, so space on 'back' of extruer is critical. to fit longer motor i had to hack the original design a bit - i did it in blender (file attached - to adjust to your likes just move the cube around so it subtracts the extruder).

this also allows motor to have shorter shaft/using normal gears

i've printed it and it's ok. i used 5 shell layers (perimeters), 5 bottom and top layers, 12% infill and 0.16 layer height to provide decent layer bonding. sliced with cura. ofcourse printed in ABS.

i think next thing for re-design is the idler - i've used two diameters of rods for idler bearing, true 8mm smooth rod for bottom one, and 7.5mm (8mm 'rated') threaded rod for top one.
only this way bottom idler 'catches' filament as top one flattens it a tiny bit. it is still verydifficult to adjust and there are also no 'inspection holes' to see if grip is correct - you have to do trial and error.