I once found a demo version of tool that implements an MSU algorithm that now seems to be completely outcasted by modern deep learning solutions like enhance or topaz gigapixel ai. Despite all photorealistic results deep learning algos give they still struggle scaling up pixel art - not just a bunch of collection of fancy pictures but also lot of old games.
The algorithm is called SmartEdge2 and its main quality towards pixel art is just a part of it, the parameter called "Fractal Substitution". I would always stick with bilinear upscaling, giving all the parameters the value of "0" keeping "2" only for fractal substitution.
Of course it's not perfect but the way it restores thing lines and even dithering patterns amaze. Before the upscaling I'd also save the source files palette and then apply it to the result easing the task of further image cleaning.
Under Windows 10 x64 this software needs a compatibity setting of Windows XP SP2 and right of administrator to run. The bitter side here is that the demo version disables the loading of images any higher than 512x512 pixels but if you do the enlargement twice, in series it works.
Here is the link: https://drive.google.com/file/d/1BWGH_s ... sp=sharing
Mirror #1: https://drive.google.com/file/d/1sCa0AK ... sp=sharing
Mirror #2: https://drive.google.com/file/d/198ta1r ... sp=sharing
Mirror #3: https://yadi.sk/d/XJjXYNHF2pDP-w
Mirror #4: https://yadi.sk/d/O6dMIFY_GKoleA
Mirror #5: https://yadi.sk/d/M3wlnpCtpTisbA