They are made from ABS, most of them should be OK or maybe even better made from pla because it’s more stiff. But think about operating temperature, don’t use pla on parts connected to a heart source,for example the parts where a stepper motor is connected to!
I can only agree on this point, i have a taz clone which is printed in PLA and after a 6 hour print the stepper bolts melted through the plastic. So now i am making a temporary solution, until i can print new parts…
If somebody here does not have the answer to the infill, you can try to check the gcode in a gcode viewer like http://gcode.ws/. And skim though the layers and check if you see any lower infill typically lines crossing or other patterns.
I didn’t think about the heat from the stepper, thanks! Do they really get hot enough to melt pla? So that means I shouldn’t use pla on the extruder parts that are near the hotend?