After straightening the header with various jacks and braceing it in place you will have to install/build a flitch beam. Usually they are 3/8 to 1/2" thick steel with similar length and width dimensions as the header. Bolts will be added through the metal plate and the wood header. The metal plate will probably be purchased at a local metal fabrication shop, and you should ask the shop to dril the hole through the plate for you. (well worth the money) The holes are staggered roughly 1/3 third of the way from the edges of the metal plate. After you remove the siding and sheathing, secure the metal plate to the header on the inside of the house with 1/2' carriage bolts.( bribe friends to come over and help. Depending on the size(length)of the metal plate the weight may be excessive. If any of this is not very clear consult with a structural engineer or architect, they can easily sketch and spec your needs for very little money. All this assuming the wood frame is not damaged in the first place. Good luck!