As there are some assets which might be required for a particular game and are not provided by Kenney, then is it allowed to make them right now in the jam and use them?

Nope, this jam you can only use the pre-made assets. No custom ones.

Except for music, sounds, fonts and (I'm guessing) font title image (as it could be considered a font), though I'm guessing since image fonts like font-awesome are a thing one could theoretically create a custom font for the missing image assets and use that, though I'm guessing that is disallowed(even though fonts are allowed).

Also Skybox I assume.