Having been standing outside, the Y Axis wasn't running smoothly. I worried, that I had to use a NEMA34 motor for driving this axis. Adding a 1.8 kg motor costing $200 wasn't on the top of my list, so I started looking into how, the Y Axis was constructed. Whereas the X Axis runs directly on the belt, the Y Axis seems to run using a magnetic transmission (for keeping dust etc. off the Y Axis belt). Designing a short arm, which could be screwed on to the Y Axis, I found, that it only required a 0.250Nm forced to move it. After cleaning and oiling the linear rails of the Y Axis, the axis ran much better and with much less force, making it more likely to be able to run using a standard NEMA23 motor.
I designed a demo fixture for a NEMA23 stepper motor with 1:3 gearing using Belts and Pulleys. PLA version seems to fit, except for a 2 degree slope, which has been compensated for prior to printing it in PETG.
Huge thanks to www.makersupplies.dk (Martin) for his suggestions on how to begin this work and material choices.