I'm not real versed in the martini rifles, but you might be able to substitute a front sight from a martini-metford or martini-enfield artillery carbine. That, in itself, might be a hard search, but there were certainly more martini carbines made than the 1500 NZicon pattern MLE carbines.

I was poking around on gunbroker, and pulled this pic from a martini henry artillery carbine. Wrong caliber, but the sight looks very similiar.

Not sure this was much help, but best of luck with the resto!