Retopo-ing high poly, un-sculpted mesh for Substance Painter

I created a model in blender. I applied subsurface modifiers before exporting it as a .fbx to use in Substance Painter, not realising that Substance painter needed a low poly mesh that it would then bake with a high poly mesh.

I don’t have a low poly mesh, and I’m pretty sure I need one to bake maps in Substance without it taking AGES.

Is the only way of me getting a low-poly mesh again is by using retopo? Would using the decimate modifier on each of the components work in this case? How would I go about doing retopo on the spherical “face”/“head”?

Here is a video i made long few month ago explaining how to reduce the density of a mesh, in the video i used the decimate modifier. start at about 4min 25sec since the video was about something else. Happy blending