Super Soaker Central

I plan on adding a trigger to my modded Rattler. I originally planned to swap out the handle part of the metal valve with something like JLS used, but I forgot about the turn stoppers. Any ideas?

You could mount the toque arm on the old handle.

I would use a solenoid, but some have reservations about electricity and water [even though the current doesnt (usually) have enough power].
If that isn't your cup of tea, I would suggest using a pulley system to reduce the force required to turn the valve.