Serration can be replaced with Amalgam Serration without losing much damage.

For Rivens I strongly recommend getting Toxin with one or two additional mods like Multishot, Critical Chance or Critical Damage, if possible with a good negative such as - Zoom or + Weapon Recoil.

Replace Infected Clip with your Riven if you manage to get Toxin, otherwise just copy my build and try your Riven in different slots.