After a splenectomy does another organ take over from the spleen?

No. There is no organ that can replace all of the functions of the spleen. Most importantly, after splenectomy, there is a risk for post-splenectomy sepsis (overwhelming infection).
No. The spleen has specific functions, such as helping to identify a specific type of encapsulated bacteria, and a vaccine is given before or after splenectomy surgery to help with this. The spleen also helps remove old worn out blood cells and helps with platelet management. A person can live just fine without a spleen.