those are called clutch head screws. You can get a set of 3 or 4 of them at some part houses. Even Lowe's or Home Depot have them at some stores. I suggest also trying Ace Hardware.
Most parts vendor's have them for around #20 bucks for a set.
Just as a tip.....if you round the head of them out you can stand a washer up on them, weld the washer, then twist them out.
Those Clutch heads were used on trucks for years.
