There is plenty out there. Heads, cams, exhaust, Carbs and intake manifolds, NOS plate systems, Suspension, Its out there. Just keep looking. Not all but some stuff for the 87-93 will work for you.
I say do a full exhaust and a upgraded carb and intake manifold for now.
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1990 LX, MAC CAI, C&L 76mm MAF, DIY ported upper, MAC 70mm T/B, BBK Equal length shorties, BBK O/R H-pipe, March U/D Pullies, Flowmaster American Thunder with Dumps, FRPP 4.10 gears, Electric Fan, 130 Amp Alt, FRPP Plug Wires, MSD Cap, MSD Coil, 14* Timing, 180* stat, BBK springs, Chrome pony's.
unfortunately there aren't alot of aftermarket hoods for your car. well i should say i only found afew for mine. anyway there are lots of aftermarket suppliers. CJ Ponyparts.com is a big one near me. i have used latemodel resto(also known as 5.0 resto), summit, ebaymotors, jegs, and cjponyparts just to name afew.
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84 gt, 351w, rpm airgap intake, 292h comp cam, comp 1.6 roller rockers, holley 750 double pumper, roush 200 windsor sr. heads, msd ignition, 9mm ford racing wires, bbk shortys, flowmaster header back exhaust, b&m short throw shifter, centerforce dual friction clutch, billet clutch quad., 3.73 rear, 5 lug conversion w/ 17x8's front & 17x9 bullit wheels rear, saleen spoiler, custom hood, custom sub frames, ALL work done personally
i have used mustang restoration before they run a good site. and i havent seen any aftermarket hoods that are made of metal. i would go with fiberglass though you kill two birds with one stone. you get your induction and a wieght reduction. and as far as other parts go almost everthing ive seen the years range from 79-93.
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1990 Mustang 5.0 LX Standard with a forged lower half, mild racing cam, aluminum racing driveshaft, bbk cold air intake, short throw gear box, aluminum H pipes with headers, roll cage (welded), 8.5 MSD sparkplug wires, MSD distributor cap w/ rotor, E3 sparck plugs
lack of after market parts for an 84 mustang 
