As Martyn said a block mounted mechanical pump driven off the cam was original figment. Either an AC or Fomoco. Nothing wrong with them, will keep a pair of Webers fed and around £25 new.
If your Mitsuba is ticking it should be working so is probably a blocked filter or line unless it has an intermittent fault. No relay is required. New electric pumps aren't expensive. I'd suggest inline filters before the pump and before the carb and see what happens. Also check for rust in the fuel tank, blow the fuel lines through and check for crud in the carb. Keep us informed!
