Unless you can find an old Psychoballistics, or Tippmann full kit, you're going custom.
Two major paths for a custom- Spring or Pneumatic ram return:
With the Spring return ram, you need only one air line going to the ram, and you can use a relatively simple 3-way solenoid (the original kits used a Humphrey H041E1). The board required for this will need to be capable of driving the solenoid and I can't go into too much more depth there. The Tippmann, and Psychoballistic boards would be capable of driving these.
With a Pneumatic return ram, you need to have a customized (or custom built) ram and, it will require two air paths. You'll also need a 4-way solenoid. A benefit of this setup is that there are large number of early electropneumatic boards that can drive this type of solenoid (some folks used old DM boards, etc). But, you will have to ensure that the board and solenoid you choose will fit, and mod the frame as necessary.
Depending on the board and solenoid chosen, you will need to fit the battery either in the grip or a battery box (custom or PsychoBallistics/Tippmann styles) and give the wires between the battery, board and solenoid, routes.
Either of these two setups will require an airpath between the LPR and the solenoid.
Once all this is done, then you can look at tightening the blowback orifice on the valve to turn eliminate it.