Are you talking about the screws that hold the exhaust flange to the head, or the studs down on the collector?
The OEM exhaust screws should be 3/8"-16 x 1-1/4", so you can try a 1-1/2" screw and see if it holds (no guarantees - I don't know how deep those holes are offhand.) However, you'd be better off using some sort of screw thread insert - like a Keensert, HeliCoil, ReCoil, or similar. This will require removal of the header - but it's also the best fix.
If it's one of the collector studs that has stripped, you can probably knock it right out and replace it using a 3/8-16 screw of the appropriate length - you'd have to measure, since I don't know that offhand, either. You will need to use a wrench on both sides of the thing when you install/remove that bolt and nut, tho...