If you want an easy path to sci-fi them up, but keeping a semi-primative feel, use 40K Ork weapons, but convert them to spear guns and cross bows.
For cross bows, take some of the bow ends from the WFB Orcs and attach them to 40K Ork guns making high tech cross bows.
For spear guns, cut the magazines off the 40K Ork weapons and drill a hole in the barrel to insert a plastic spear. For the larger 40K Ork guns, use WFB Orc spears. For the pistols, use human, elf, or goblin spears. You can either assume they are all spring loaded, or fashion some gas canisters and attach them where the magazine was cut off.
The Orc you have pictured could be reaching for another spear. Just fashion him a large "quiver" on his back with a spear or two in it.