I realised that I don't like the current Scribble Rreign look, I was thinking about uploading the old version to the Custom pet image.

After looking at the rules, I'm not sure if its allowed. If it isn't, how can I get the original look back?

If you make some changes to the original image you should be able to use the old art as your custom overlay; I've seen a Custom Chibi Pet a while back that used the old art with a couple of dark spots added for example.

Thats the problem, I don't really want to change the original image and the look is what made me want to adopt him in the first place.

You'll have to just use a profile overlay for that, unfortunately. I'm pretty sure the Custom Overlay rules clearly says "No unaltered site art" or something like that.