The head on the original kit is not as dreadful as one might expect from an 80's Gundam model. However, it's a standard ZZ Gundam head, not at all accurate for this version. The Sentinel version has the trademark Katoki long head with pinched face, a look I really wanted to replicate here. I was worried I wouldn't be able to pull it off, but after extensively reworking the kit parts I got the look I wanted.
I started by cutting away the face and neck (originally it's just a 1 piece front and back head assembly, no separate face or neck as on modern kits). Nothing worth keeping here, I tossed the face. I lengthened the head by laminating a few sheets of plastic between the front and back parts, then Bondo filled and filed to match the new shape. The cheeks were extended with sheet plastic as well, it's basically a scratchbuild after all is said and done. The new face is sculpted from epoxy plumbers putty with styrene eyes, chin, and nose. Not an easy job, this bugger is tiny!
The head required all new details to match the line art as well. I was able to save the Mega Beam Cannon on the forehead, enlarging it by boxing it in with sheet plastic blended with Bondo. I added a tiny crimp bead to a hole drilled in the center for greebleage. The head spikes are new, sheet styrene with brass rod antennae.
The back of the head features a louvered vent look. I cut an opening in the head, then beveled the edges of sheet plastic and laminated a stack of them to replicate this look. My favorite head details are the tiny rectangular vents on the side of the head. These are about 1mm x 2mm! I used the tip of a precision screwdriver to make the vent hole (really an indentation) in some thick styrene sheet. Then I cut the rectangles out around the indentation (very sharp knife!) to create these tiny vents. I must have made about 20 before I got them right! They fit into rectangular recesses I made in the head using the ever present Bondo Squish trick.
I tried a bunch of methods to make the head vulcans, eventually giving up...they're just too small! I used an Alps printed decal on the finished model, I think it came out looking pretty good (see the finished head). Here are the completed head parts before priming.
I sawed off the front and back of the outer shoulder armor and discarded the rest. These were combined with some plastic sheet to make a box that fits over the kit inner shoulder. I built a new pauldron from sheet styrene as well. The large vent on the outer shoulder armor was based on a rectangular piece of plastic sheet, with beveled edges formed from Bondo. Louvre textured styrene completes the vent detail. A quick "Bondo squish" action added the recessed vent detail on the front and back faces of the armor. Below you can see the original compared with the modified armor, and a detail view of the large vent (note the Bondo squish vent in the right pic).