For me if Goku has enough time to train he could beat Superman. We all know that Superman has a limit of his powers. For saiyans we know that after battle they've become stronger and when they train they get much more power than the human beings. In the beggining goku moved a whole mountain just for 2 days and he was a kid. Nowhere has been said saiyans has limit or not. If they do why they still havent reach that limit? Their story start with power level less than 1300. Goku can easily go to the outworld where he can train whatever he want with the help of the Kais. But however Goku's gona fight Superman in his current form and he's gona lose. In the second fight or any other fight Goku's gona win because he knows the Superman's powers.