Hi,
I've recently toyed with ESRGAN and SFTGAN to upscale textures in old games. This is how I ended up releasing a neural upscaled texture pack for Quake 2 a few months ago
I've gotten pretty decent results above by also using a denoiser on the output – I used GIMP's noise reduction filter for this. It tends to look better for cartoony textures overall.
Some textures in SuperTuxKart (but not all) tend to be a bit blurry, especially on higher resolution displays. Would the developers be interested in me providing upscaled variants for those? I've noticed a few in specific tracks already, but having a more complete list would be even better. Of course, we need to balance this with the distribution size, but we can probably get substantial improvements in graphics quality with a moderate increase in download size.