<p>When exchanging the mainboard of my Ultimaker Original, I found the new (unoriginal) mainboard to have a condensator that is quite a little higher than on the original, which means the fan duct doesn't fit any more.</p>

<p>I read somewhere that those A4988 stepper motor drivers don't have to be actively cooled, but a heatsink will be sufficient:</p>

<p><a href="https://i.stack.imgur.com/oJmlL.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/oJmlL.jpg" alt="A4988 stepper motor driver with heatsink"></a></p>

<p>After testing the theory I found the heatsinks to be really hot, but not too hot to touch. Sadly I do not have any means of temperature measurement other than my fingers...</p>

<p>How hot do they normally get, when should I abort printing and look for a different solution?</p>

<p>Are there any good solutions other than the original cooler &amp; cooling duct?</p>

This may not be your cuppa tea, but if you're willing to learn to use
OpenSCAD
or already know how, there's a
Thingiverse
post that appears to directly address your objective.
Correction, this particular post on Thingiverse consists of a series of Python files, of which I have zero experience/qualifications. It may still be of value, if you are Python capable.
Another resource that is strictly OpenSCAD is from Eric Buijs, a rather talented 3D design person. His YouTube channel has a number of useful tutorials for both OpenSCAD and Solvespace.
This video
in particular describes applying a flat object to a curved one using OpenSCAD, resulting in a lithophane.
As I created this answer, I did not re-watch the 12+ minute video, but I recall how he explains clearly how the program dissects the surface into a number of flat panels and then superimposes the image on each segment. From this presentation, I suspect one could expand to a full cylinder.