Matthew wrote:
Sometimes Ninja Force heads will fit without mods (although they may be a little wobbly).
Here's my process
1. Cut the neck ball off of another figure
2. Drill a hole through both the new neck ball and the head
3. Take a piece of metal wire (think metal construction wire that comes in a small spool) and run it through the neck ball into the head
4. Place the assembly on the body to make sure the height is correct.
5. Add modeling putty to fill in the gaps. Let it cure.
6. Sand
7. Paint.
This is the method I use. It has helped especially when things borke on the mail or moving being able to reattach it quickly.
http://wiki.joecustoms.com/wiki/Pinning_is_Winning