This is related to how complicated is the model. If it's very basic and simple, just lock imported model and by snapping to vertex do hand retopology. You can be sure that mesh is correct in this case. Other idea is to first clean the model (connect detached polygons, group rest, etc) and use a subdivide modifier with retopology. It can generate "ok" mesh. Later on you have to probably fix some part by yourself, but it can make all the process faster.
Whatever you do, next you have to apply texture again. This can be tricky, and I always make my own material by using RizomUV software (best for working with uv) and substance painter. But if you don't want to do it, I think you have to do retopology base on the how the mesh looks like before (I mean, keep the same group). I hope this can help you.
