Attaching a resin head to an Obitsu body is not hard but it does require a little modification to the Obtsu neck piece.
Remove the neck piece from the Obitsu body.
On the inside there is a small Phillips screw. Remove this screw. This will allow you to remove the interior parts of the neck piece.
You will need the body of the neck piece (the large hollow tube part). Put the other neck piece parts away in a safe place in case you wish to restore the neck piece in the future.
Next you will need a piece of stiff wire. You can use a piece of wire snipped from a coat hanger. Cut a piece of wire about 5/8" (1.7cm) long, the diameter of the outside of the neck piece body.
Now you will need to drill a pair of holes in the body of the neck piece. The holes will need to be the same diameter as the thickness of your wire. You might have to take a piece of your wire to a hardware store to find a drill bit that is the right size. You should be able to buy a single drill bit (not a set) that shouldn't cost very much.
Measure down from the top of the neck piece about 1/2" or 1cm. The top of the neck piece is the end with the angled collar that sticks out of the neck vinyl.
At your mark using your wire sized drill bit, drill a hole through the neck piece body, from one side to the other. Continue drilling till you drill through the second side of the tube. This should essentially be across the diameter of the tube.
Now insert your wire into one side and through to the other side of the neck piece. Depending on how well you matched the drill bit to the wire diameter, this may be a snug fit. If so, tap the wire in gently with a hammer. If the wire is loose, a drop of super glue on each end will keep it from moving around.
Now you should have a neck piece tube with a piece of wire running across the middle.
When the glue drys (if you used glue), thread a piece of stringing elastic down through the top of the neck piece, then back up through the neck piece so both ends stick out of the top of the neck piece. You want to make sure the elastic loops around the wire so it doesn't pull out the top. The length of elastic will depend on the brand of head and how the elastic normally terminates in the head. Cut the elastic a bit longer than you think you will need. You can always trim off the extra
Re-inset the neck piece on the Obitsu body and thread the elastic through the neck side of the resin head. Finish off securing the elastic and head as you would normally.