For a good injector upgrade for the 4ZE1 manifold you can use Mitsi VR4 or Evo injectors.
They are 510cc and there is little mods needed.
You need to bore out the 4ZE1 fuel rail a tad and get different rubber bungs for the manifold. You also need to replace the end cap on the injectors with a single nozzel as they have twin (for twin valve).
Injectors for 4ZE1 manifolds
mazda 13BT series 5 injectors fit as well, in the later model single throttle body 4ZE1 manifolds. The stock fuel rail is a tad tight to fit the injectors in, so I put some wet and dry on the end of something round (can't remember) and enlarged the I.D. of the fuel rail a tiny bit, then they squeezed in. They're 550cc, and everything else fits in beautifully, their nippon denso injectors too so the stock injector plugs fit too.