Check out these useful The PolyHatchet tools for MODO that can slice a mesh at the origin, remove half of the mesh and mirror over the side of your choice. The toolkit can remind you of Slice Mirror Merge in ZBrush.
These tools are free to artists that have purchased the Hatchet Collection for MODO. If you own this Kit, this new kit is available to you free of charge and can be downloaded from your Gumroad Account.