Glue back the missing spikes and smooth out any cracks and fracture lines, with watered down modeling putty. Then cover it with several light passes of black acrylic paint. Preferably a matte one.
Unless You don't have the missing bits, in which case it's a bit trickie,r but doable. My Sauron helmet was missing a few pieces and I had to build them from scratch. The spikes are a simple shape, so depending on Your modeling skills, You could carve one out of putty.
There's another way hovever, one that I've used on my Sauron helmet. The helm is simmetrical, with each spike having it's mirror version on the other side. So You could just make a simple, ad-hoc mold of the non-cracked spike, cast a duplicate out of it, cut it to lenght and glue it on the cracked one (and smooth the fractures with watered down putty). Its easier and faster than it sounds. Since the spikes are a super simple shape, You don't really need any fancy mould made of of proper materials. Just take some sort of cheap plasticine (anything that dries relatively hard, really. Even some types of kindergarden, child plaster will do), lube up the spike with oil or vaseline, so that it won't stick. Cover it in a thick blob of plasticine and wait to dry. Once hardened remove the mould and simply make a cast using any sort of casting material You can think of. Cut to lenght. Glue. Smooth out. Paint. Done.