Replacing the jack stud/trimmer isn't terribly difficult. I've done it many times when lengthening headers to increase opening widths. That might be the simplest fix if the king isn't also warped.
- Cut the nails penetrating each end with a reciprocating saw - don't cut deeper than 1-1/2" so trim covers the cuts in the drywall
- Remove any nails in the face of the stud
- Find and remove drywall screws penetrating the stud
- Run a pair of longer screws into the stud near one end, leaving them 3/4" proud
- Pry the stud out with a bar or large hammer on the screw heads
Otherwise, run some 3" screws in to tighten and straighten the jack as much as it'll go, then rely on your corner bead, which can easily take care of a 1/2" twist (1/4" at each end), for example:
- Install your drywall as normal
- Set the innermost (opposite) ends of both corner beads
- Use a framing square to set the opposite ends square with the previous set opposing ends
- Check the bead on the wall face with the square to be sure the bead is slightly proud of the drywall plane. It's easy to pull it in too far, essentially creating a curve in the wall.
Be aware that the second approach might reduce your finished opening width that 1/4".