Pre-WW-II cast aluminum has impurities that
make welding it difficult. You could open it
up from the back side, not going through to the
surface, and use the aluminum brazing rod but it
won’t flow through so the remains of the crack
would still be visible.
Switching them out when you want to run it is
probably the best option till you find a good
replacement.
Tubs
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.