Most FS2004 add-ons will work in FSX as is. The only real difference is *native* FSX add-ons use a .dds format for the texture files (There's a .mdl difference too, but fs2004 .mdl work in FSX). But on the whole, .bmps work just fine.
There may be gauge problems too, but you can swap them from a working panel, usually.
Are you looking at a specific vehicle, or just "In General"?