You can try this.
Drag a marque selection to select all the vertices of one object. Edit Mesh and use merge command with extreme low value like 0.001.
Next step go to Mesh and run Remesh. the lower the value the denser the triangulated geometry.
Then run Retopo. do not check Keep Hard Edge. Max poly count 3000 to 5000. Play with this value. Every time you do something the Retopo will start calculating from start again. So have patience once you are happy delete history.
It will be not 100% perfect. But little more editing edge loops can make it better. See the image, I converted those pieces to all quad.
