You should use waifu2x to upscale the graphics in OpenRCT2. It has incredible upscaling capabilities because it uses a neural network. You can even just use it to remove noise without upscaling.
Try it out here!
Pictures:
Upscaling using medium noise reduction, artwork, and 2x scaling.

Noise reduction highest with no upscaling and srtwork settings.

Presume it would potentially solve 775
Duplicate of https://github.com/OpenRCT2/OpenRCT2/pull/2328.
See https://github.com/OpenRCT2/OpenRCT2/pull/2328#issuecomment-157491637 in particular.
It seems like the other issue takes the approach of running waifu2x at runtime. Wouldn't it be more efficient to upscale all of the sprites once and then use them?
We can't do it, at least not yet.
Note that there are now better upscaling neural networks: https://github.com/xinntao/ESRGAN and https://github.com/xinntao/SFTGAN
Can you provide a sample image?