Summary

This neck joint allows the head to roll from side to side as well as move up/down and left and right.
https://youtu.be/GDpuJFCJ2hM

Instructions

Print;
1 - Neck Joint Upper
1 - Neck Joint Lower
2 - Piston Adapter S
1 - Piston Adapter F
2 - Servo Holder
2 - Servo Pivot
2 - Throat piston V2 ( from InMoov website/downloads)
2 - Throat piston base V2 ( from InMoov website/downloads)
Requires at least 1 servo - I used a MG995 but any servo this size will work.
Before beginning the assembly make sure all of the parts move freely when fitted together. The parts should snap together. Put a little grease in all of the joints.
Cut the Neck V1 off at the first groove and fit the Neck Joint Lower part into it. The narrow groove on the part faces the front. Secure the part in the Neck with a screw or glue.
Replace the original Neck HingeV1 with the Neck Joint Upper and attach with 3 screws.
Make sure that the ball on the Servo Holder fits tight in the Servo Pivot but still moves freely.
Mount the Servo Pivot on the inside of the irregular shaped hole between the Homeplateback and Claviback, with a couple of screws.See picture.
Cut the ThroatPistonBases so they are 3" long and trim the base so It is 7/8" in diameter. Attach this on your servo.
Cut the attachment point off of the ThroatPistons and attach Piston Adapter S to the unthreaded end - don't permanently attach them yet.
This setup requires the original ThroatPiston and Base V2 that lifts the head up and down. Cut the attachment point off the ThroatPiston and attach the Piston Adapter F to this one.
Set the servo that lifts the head up and down to 90 degrees. Attach the throat piston to front attachment point on the Neck Joint Upper part.Move the head up and down by hand . Once you have the travel set for full motion permanently attach the Piston Adapter F (use a small screw in case readjustment is required)

You will have to remove some material from the Homeplateback and claviback for the ThroatPistonBase to fit.
Mount a servo in the Servo Holder, set the servo to 90 degrees and attach it to the Servo Pivot, attach the Throat piston to a side attachment point. Move the head side to side by hand, again once the travel is correct permanently attach Piston Adapter S.

This system will work with one servo. If you choose to do this, then the other servo is just a dummy servo. Strip the inside of a wrecked servo leaving only the gear that sticks out of the top and attach the ThroatPiston Base to it. It is there for show.

If you want to have 2 servos driving the movement. Wire them up basically as the servos are connected for the Lower Stomach. Don't remove the Pot and connect the power leads so that the are reversed. Also make the slave servo so it will rotate 360 degrees by removing the stop on the gear inside the servo.

That's it! Power up the robot and see how it works. Some fine tuning may be needed, Once you have everything dialed in, put a small screw thru the Piston Adapters into their attachment points. This will keep them from twisting off.
See the pictures.
Update July 22, 2019 Gael has reworked my design. It's more robust than mine - I would recommend using his design, I do now. Check it out here;http://inmoov.fr/inmoov-stl-parts-viewer/?bodyparts=Neck

Articulating Neck Joint for InMoov