I would definitely try to push the wall in. If you can't push, pull them together with a cumalong or fowler until the wall is straight. You should leave old 2x4 in place, this is fastened to plywood. Pull diagonal brace off, then screw 2x4 back together through crack with 3" screws, then add another sistered to it with screws, 3" again, this will repair 2x4. Now the main problem...this board is not for holding the wall, it is for roof support. What you need to do is buy some 2x6's, enough to go across garage end to end. If you have to use two then overlap them at least 4'. Fasten this on top of now straight wall on opposite side of diagonal brace. Screw through this with a few 4" screws, into your 2x4's and also diagonally down into top of wall. Then fasten to house side in same manner, or screw a cleat to wall. If you need help on this side just ask, we're here. Good luck......