To re-upload the model and repaint you have to ask the maker of the model.

The repaint alone I'm not to sure

Every model designer has their own requirements. Some specifically don't want to be contacted (Kirk Olssen for example) -- most allow as long as credit is given --- some (like Posky) say one thing but actually do another ... and one or two treat it like their creation was the cure for diabetes. (fortunately they are very rare and you will usually find the levels of seriousness are inverse to the talent of the designer)

It is always good to communicate with the model creators if it is possible (many times it is not). And if the readme states permission required it is always a good thing to ask ... there are usually further benefits like building a respectful relationship.