Hi,
I happen to have had some experience making fibreglass copies of medieval helmets, so may have some useful tips.
First thing you need to do is make sure there is a full seal on the helmet you have worked on and cover the helmet surrounding the area to be moulded with a pvc tape. You will need to waste some silicon rubber now. Make a block of silicone around an inch (2.5cm) thick and about 4 inches( 10cm) square. This will be cut into cubes once cured .
Then mix a small amount of silicone rubber and paint it on to your sculpt with about an inch of bleed around it. Not a thick coat as you are just creating a smooth skin at this point.
mix up you main batch of silicone and add a silicone thixotropic gel to the mix. This will turn the usually runny silicone into a spreadable paste that can be put on thick. Once you have spread it over your sculpt, take the cubes you made earlier and set them in the thick layer of silicone paste so that they are evenly spaced over the mould. The purpose of these is to act as anchors for the next process, so they should protrude at least half an inch and be evenly spaced in rows around the curve of the mould.
Next step is unpleasant but necessary. When the silicone is fully cured, you will need to apply resin and fibreglass to the outside of your mould, making sure to properly encapsulate your silicone anchors. this fibreglass should be tough and durable 3mm (1/8th inch) should be sufficient.
The curved nature of your mould will help the anchors stay in place in the fibreglass backing, stopping your mould from warping. Fibreglass is handy because it is rigid but does have some flex so that you can remove the mould from your sculpt when the mould is dry.
When it comes to casting ,probably your best bet is fibreglass re-enforced polyurethane. Some thing with a 30 min pot life will help when it comes to laying down the fibreglass. As mangozac suggested painting a layer of resin first is essential, this first layer should be dry before adding further coats with fibreglass.
The benefit of this method is you can make the cast as thick as you like and it will be very strong.
the negative is that you will need to clean up the back of the cast.
The other way is to make a negative mould of the front of the helmet by pouring silicone into a mould of a basic helmet frontage ( not forgetting mould release agent!) , to form the back of a two part mould ( expensive!) you would get a better fit, but its a lot more trouble to go to.
hope this helps