I am not quite as negative on the possibility of repair for this. It will be difficult, but if you are at the end of a very long supply chain it may be easier than replacement. You will need a steady hand, fine tweezers, a microscope and much patience. No guarantees but it can be done. You will have to figure out how to install the cap with brushes onto the commutator without mangling the brushes. This can involve things like very careful shaping of the brushes, fixtures and multiple tries.
Before you embark on this think about why it failed in the first place. The small cavity under the brushes captures the end of the motor shaft and forms a bearing. Most likely it has worn to a much larger diameter than its original. So the motor did strange gyrations and bent the brushes and threw the end off. A repair would have to correct this problem also. Depending on the tools you have access to this would range from tricky to impossible.
A purchase may look much more attractive after thinking all of this through. Small motors like this are inexpensive at their source, and might not be that bad after they make their way to you.
Finally, your concept for supplying one side of the power to the motor through a spring contacting the case is feasible. Since the end cap is non-conductive you could put another metal cylinder on the other side with a second spring to get the other side. This is very different from normal brush design and will have life, friction, contact resistance and other problems but might well serve a non-demanding application. That does not solve the problem that this motor is not design for or compatible with underwater operation. It might work in a non-conductive fluid like a light oil.