Is there any tricks to remove mud flaps from fenders?

I have an '86 Honda Fourtrax 250 that I am trying to get ready for Duck Hunting riding on the muddy levees. While shopping, I found a new aftermarket rear fender assembly that was returned because of no tool box. Shop owner was stuck with it so he sold it to me cheap. But I can't get the black mud flaps off the old fender assembly of which I need for the heavy mud I encounter. The strait slot screws are rusted to the small nut/clips under neath that hold them to the fender. I used WD-40, tapped on them and even tried putting vice grips on the back side. Is there a special tool for these type of screws that more pressure can be put on them vs. a big screwdriver? Or do I have to drill all of these screws out? What is a good Screw/nut replacement to use? OEM screws and nut clips will cost more than what I paid for the fender assembly. And this atv is just used for hunting, not show but appreciate any ideas from you.
thecruiseguy