This piece is two parts, the adapter and the hose connector. The adapter diameter is 1 1/4 inches but I'm not sure about the hose connector diameter. More about that later.
This is the flange for the pump. It's diameter is 2".
The piece screws together like this. We need a piece that has 2" treads on the outside and threads down to 1 1/2". This is the difficult part to find.
It will assemble like this. If we can't get a piece to bridge the adapter and flange, we might find something to bridge the hose connector and flange.