How do you take them off without damaging them?
1) Does the plate need to be removed in place or at all? i.e if it is on the cover plate of the fuel injection housing, just remove the cap screws to start. or leave the cover and plate intact.
2) If the plate is on sheet metal such as a seat back, the drive screws shank usually can be found sticking out a little. A sharp rap with a hammer, a further nudge with a center or prick punch, and final removal with a small pin punch, or vise grips on the head.
3) If you have a Bridgeport mill, you can drive out blind drive screws from radiator plates by drilling a small depression in a bit of metal to locate the head, and then drilling the casting from the reverse side until the end of the fastener is found. Then drive out screw as you like.
4) If you are haunting the scrapyard as I do, and time is of the essence, sharpen your cold chisel to rap around the head of the rivet so it will not slide off. The included angle of the chisel can be asymmetric to the center-line of the chisel to make the chisel lie closer to the plate. Please use a thin shim between the chisel and the plate. Leather,teflon or nylon sheet come to mind and can be had at your local gasket maker, or try McMaster.com. Then limit your hammer blows so when the shank breaks it will not carry away the corner of the plate. Or try placing the chisel outside of the plate corner and drive toward the center.
5) The drill jig--Get a 1 x 3/16 x 6 flat bar, drill a suitable hole 1/2" from the end and centered. Countersink the hole to locate the screw head. Hold jig firmly onto screw-head and drill center of head with a new split-point drill deep enough to weaken the head and reduce the need for excessive hammering
I have used every suggestion listed with good results, the only trick is to use the most appropriate method or combination of methods
It is madness to just walk up and start banging away with a hammer and chisel and wonder why the end result is always the same.