Parts have been modified to use a T8 Lead Screw vs. a 5mm threaded rod. I had to move the T8 mounting holes about 3mm away from the 8mm smooth rod. The X carriage has been stripped of extruder and hot end parts and is now just a flat mounting plate for tools. The X carriage also has a mount for an 8mm inductive bed sensor. The pen holder assembly uses parts from a DVD drive. The pen holder rides on two 3mm smooth rods and no springs or rubber bands are required, just the weight of the pen and pen holder is enough to get a nice line. The clamps are sized for a fine point Sharpie. Use ABS or PETG for parts that hold stepper motors, otherwise PLA is OK.
Here is where I found the Prusa parts that I modified: https://www.prusa3d.com/prusa-i3-printable-parts/
See it in action at https://youtu.be/qLj5sCHtTaw
Print the pen holder clamps horizontally so they will have enough strength to expand aroung the pen.