Missing Derelict Parts

I reseted the game just so i could replay all again for fun, you know beat all the bosses again, get all the derilicts again, start from zero and stuff but then i collected all the parts from the bel air (One of my favorite cars from the game btw) then i came back to the garage after COLLECTING the last piece on that jump on the desert near the gas station, then i look at the derilict and said i still needed one part then i tought okay maybe just i glitch or i did it too fast i'll come back and get the part again... the part was missing and i was unable to get the mac attack as i did the last time i played the whole game... i could just reset and replay but i would lose all the designs i made for all my cars this time (Which i'm actually proud of) as well as rachel's 370z and The M3, Mitko's GTR, Faith's Raptor, i would lose everything if i reseted and i dont feel like playing the whole thing again to risk not getting it all...