I think the oval shape was the result of an injury somewhere along the line after it was molded, either at the factory or by a PO. If I was going to try to straighten it, I think I'd take a piece of hard wood, cut a round hole in it the same diamter that the tube is supposed to be, then cut the wood in half (maybe cutting an 1/8" from one side. Use one side as the anvil & the other for the top "punch". Put the manifold in the anvil with the "widest" direction vertically oriented. If you have a hydraulic press, use it to ease the blocks of wood together (if no press, put it all into a vice & try to crank the vice closed......& if no vice, use a hammer). Don't try to get it perfect, as aluminum doesn't bend a 2nd time very well.
Mike Sal