monza 160 jnr oil leak

I have almost finished a restoration on the monza jnr but have discovered an oil leek from the cylinder head. After opening this up it seems to be coming from an oil ‘nipple’ situated next to the rear left cylinder head bolt. Is it possible to remove this and if so where can i buy a suitable washer? If there are any monza engine specialists out there, It would be great to pick your brains.