The Quad Draw tool in Maya is great for retopologizing a model. You basically draw a new mesh on top of the (probably way too high density scanned) mesh, Here is a video on the basics.
To start drawing quads on top of an existing mesh
- Select the surface you want to re-topo (draw on)
- Make live (right click – make live, or magnet icon [plain magnet])
You can see which surface is live on the top icon shelf, next to the snapping and live magnet
Quad Draw “Hotkeys”
- LMB Click > Create vertices
- Shift + LMB Click > Create Polygons
- CTRL + Shift + LMB Click > Delete Components
- CTRL + LMB Click > Insert Edge Loops
- MMB drag >Move components (tweak)
- TAB LMB drag > Extend (Extrude Edge)
- TAB MMB drag > Extend Edge Loop