Is the boat or trailer sitting level from side to side?
But to answer your question you can't rebiuld just one side. Rotate it two cogs and make sure the other side is on the same cog (from the same reference point). Hope that makes sense.
You need to have room for the axel to work. That is, you need to have clearence between the fender and tire.
In light of MOLDY's bad news, you can probably get a direct replacelment axel from Six Roblee's