Beginner help with mirroring and fusing a object made from a STL file

[Anonymous]

Hi! I am helping a friend with a pinball project and need som help with how to make this part whole again, so I can 3D print it. So I have had the broken part 3D scanned and got me a STL file to work with. From what i have learnd the best approach to fix this part is to make it to a object from mesh and mirror it and then fuse those into one object.

Gotten so far that i have converted the mesh to a object and mirrored it, but my big issue is to align them so they become fused, thinking it has to do with the mirror plane but not sure how to solve it, any help is welcome.