some ideas are to either make a pepakura bio and do the 1st 2-4 coats of resin then snip / dremel it to put the parts together and keep working using resin and bondo...... or you could put this on a lifecast and sculpt a whole new mask with both parts. I think for your needs you could even start with a cheap generic(ish) airsoft helmet and sculpt right over it then make a mold. You could then make parts to resin and bondo to a mask, or even (if very careful) scuff sand it and attach the mold to the same model helmet minus clay and tape the seams then pour resin into the mold so it bonds to the helmet. next would be preparing for paint. This all depends on your experience and comfort level with various materials. good luck with whatever you decide.