So keeping the core units in place I'd offer the following advice.
Drop most of the upgrades on the Trygon, he doesn't really need them. Just go stock for 148pts.
The Carnifex is very loaded up on stuff it doesn't really need. You could probably drop most of the upgrades and just give him the guns and enhanced senses. With such a low amount of points you want to maximise how many units you can bring IMO. I'd certainly drop the bone tail, toxin sacs and spore cysts on him. Personally when it comes to fexes I tend to lean towards the screamer killers. Anyway, dropping the upgrades makes him 109pts.
Another option is hive guard. A unit of 3 with impaler cannons is 144. Together with the trygon, Broodlord and Stealers that would be 647.