Thanks everyone!
I just started to remodel the thing by hand. Since it’s mostly altered basic shapes this can be done in a fair amount of time.
Guess the retopo would be more time consuming. I tried different retopo tools and workflows but it seems to not do the job as needed.
What IronicParadox told about ZBrush sonds nice, but unfortunately I don’t own ZBrush yet. Might chceck it out in the future but for now I’m happy with blender + substance.