Strengthening an old garage
 

Sorry, no pics yet. The old house I'm buying has an old garage to match. It only has about 4 17' 2X4s tying the walls together one is sagging so much it's been propped up with a couple of 4X4 posts. What is best way to firm this up? Was thinking of 2X6 beams and maybe additional bracing as needed, or should I call in an engineer?