If your dad paints cars, he knows about flex additive. That is added in small amounts to paint and clear coat. Add a little extra to each and it should be fine for quite awhile. I would not prime plastic to paint it. Use several coats of the paint to blend out the scratches and old color.
I have not done this on a quad yet, so dont take it as a given. It still may not work. Get an old MX fender or ATC front and practice and bend it around to see if it stresses the paint.
2004 TRX450R
Previously owned:1988 250R, 2000 400ex,1999 440ex,1994 300EX,1988 Tecate4,198? Tecate3,1984 ATC250R,1998 Blaster,1988 Blaster,1991 LT230(junk),1989 lt250S(4 stroke),numerous dirtbikes, streetbikes,sleds..and wifes 1998 3004x4