i have potentially developed a method of creating brand new pens for ALPS plotters only reusing the old metal casings (possibly able to 3d print those as well, but tried an easier method first)
ballpoint is taken from inexpensive new 0.7mm fine tip pens, held in 3d printed cap
fourth prototype, now attaches a small length of the ballpoint ink tube as well. unfortunately the larger hole to accommodate the ink tube made the tip a little crooked and loose, so it still needs adjustments. because of it being loose it doesn't always properly touch the paper
looks like drawing is still an issue even with the DTR->CTS improvement. i think the printer doesn't tell the computer to hold for long enough while drawing circles, only lines for some reason
after a while the pen stopped working, i think the ink still isn't being fed properly
still, i think this is a good start. and i can still buy more of those fine point pens since they're much cheaper than trying to source new old stock for this plotter
right now it still needs refinement (i think after the ink in the ballpoint tip alone dries out, i don't think it reaches the cotton inkwell to replenish, and i don't think these tips work with the same capillary action the original ones do) but after a very quick test it works