This would be for an extract porter. I don't want to have to order it if there is a more common substitute.
What crystal malts do you have access to? I would use something like crystal 120 if it the lovibond rating is otherwise unspecified in the recipe.
english dark is close to 80L
I can get 80 or 90L I think, as well as 120L.
Either 80 or 120 would be fine in a porter but I think english dark crystal is 75 so 80 would be your closest option.