@jan i tried that a lot with high res scans from sketchfab, cleaning it up, always ended up retoplogizing by hand with build in tools (and later with retopoflow) or rebuild it lowpoly and then baking the texture from the original to the lowpoly (where possible normalmap from highpoly to low poly). works fine for things like gravestones e.g., but for complex things its a real pain and often faster to do it by hand :). but thats just my experience.