With so many NC Miatas getting K swapped, you should consider complementing your swap by replacing that old faded coolant reservoir with a new Koyo aluminum reservoir expansion tank.

Features:

• Direct replacement for the plastic
  OEM unit - no modifications, additional parts, or cutting
  necessary
• All-aluminum, hand-welded tank
  eliminates risk of plastic cracking and
  rapid loss of pressurized coolant
• Retains OEM Mazda expansion cap
• Includes side gauge for viewing
  coolant level