1. Cut along the imaginary line with a dremel
2. Glue rare earth magnets to matching positions on each piece (make sure you consider polarity)
3. Bondo the edges and sand to eliminate any gaps and make it look like it did before
4. Prime and Paint.
I wouldn't recommend a dremel. It's a painstaking process of using an x-acto knife and going over the cut line many, many times over the course of a couple weeks. You want to minimize the amount of material removed so it is a seamless join between the two surfaces.
I know that some people cut up the blade from a jeweler's saw, hobby coping saw, or even a thin hacksaw blade to make a miniature saw. It needs a thin profile, regardless of how you make the saw.
The band across the top becomes a very fragile piece of the face. It tends to break every once in a while. If you have trouble getting your head into the helmet, try cutting out the ring at the bottom instead of this mod. I considered this mod for my clone helmet/costume, but I'm building one with binocs. I don't think heavy binocs + a face-off mod would go together very well at all...
What necro said, is spot on....although you could do it either way from what Ive seen his way would be better.
I have seen this done with the back cut off instead (or the bottom ring method that Necro mentioned). Might be a little better option for you and to minimize the attention focused to that area. Either way, good luck with it!
True , don't use a dremel , the cut it makes takes away too much material.
There are thin saw blades that fit in the same holder as x acto blades.
Hobby Mini Razor Saw Set
it was a pain , but well worth the effort , but when taking the back off , you can get away with some errors...