I'm surprised 1/2-20x2.5 is fully threaded, but since you ask, McMaster-Carr have almost anything you need: http://www.mcmaster.com/#cap-screws/=3m4kqf

When you replace them, I suggest you get the standard version here - and if you scroll down, there is a drawing: http://www.mcmaster.com/#92198a439/=3m4l9d

Me, I would use 180,000psi ARP bolts there