Single stage enamel can be blended weather it's a metallic paint or not. Stay far far away from anyone who says it can't and only does base clear paints. It's a lost art anymore, the paint is getting harder to find but pretty much any acrylic enamel will work, doesn't have to be PPG. DuPont, Acme, Sherwin Williams and several others make single stage acrylics. The ingredients and recipe should the same regardless of brand, just different paint/part numbers for the mix. The trick with metallics is to get the same size, style and type of metal flake and to have it lay down the same as the factory paint. Finding the right gun/nozzle may be tough, but a real painter will know what to do to make it match. I would buff the car first to get the old paint up to snuff. Best of luck, very few people will even want to attempt it but it will be well worth the effort when done correctly.