So I see someone does this in Maya but I don't know Maya.
The idea is this

Setup
I have a standard rigged head mesh from metahuman, let's called it head S.

And I have a custom scanned/photogrammetry/sculpted head mesh(or even opened front face mesh only). Let's call it head C.

Their tricks
Morph, or project mesh S to C so that S match the shape of mesh C. (or at least its front part, the face match the shape of mesh C)

Result
You get a rigged head that looks exactly like head C.

how to do this in Houdini? Can we use Ray node? I tried but I got uneven and wild floaty polygons around the edge of the mesh's open border.

Houdini is expected to provide topotransfer in a future version.
You can transfer topology of headS to headC, and then apply blendshape.

Topotransfer has been available since 18.5: https://www.sidefx.com/docs/houdini/nodes/sop/topotransfer.html [www.sidefx.com]