Just my two cents .
I have used awful-ly made crank spindles. They are just slightly wrong , the taper or lack of it the bearing support spacing, bent, erc.
These guys are not newbies when shifter kits are used.
They sell three different crank spindles that are properly sized and made.
134 mm, 135mm, and 158 mm.
Sick Bike Parts LLC - Performance parts for your motorized bicycle
Thats dax also sells a good one.
From my experience it makes a difference if the engine driven sprocket , and crank drive sprockets align perfectly. As well as the crank to freewheel setup at least on the bottom . From the derailuer to the crank sprocket.
I do not like pulling off on the sidewalk to put it back together at all.
Staton uses two guides for his unusually long drive chain.
I use a strong spring loaded idler wheel with a rollerskate wheel.