So it's already an assembled build and you want to reinforce it then?
If it's a work helmet, slushcast the inside with some thinned bondo (google 'rondo helmet')
If it's a wear helmet, then fiberglass is probably your best choice if you want to not kill your neck. Paint it over to protect from any possible fumes though.
But as Fett 4 Real said, all of these are still going to be brittle though, it's just the nature of these polyester resin products; it's why these are usually work helmets and get recast into urethene. You can order some specialty products that are less brittle, but they're going to be expensive. I'd just try to keep as much natural Sintra as possible, because at least it's light and flexible. Maybe just spam extra thin CA glue at all the joints and try not to drop it. Or hot glue.