Heering Cherry liqueur is made to an 1818 recipe from Danish Stevns cherries, harvested in August. The fruit is crushed with the stones, giving Heering...
I've never tried Cherry Heering and don't want to add seldom used bottles to my blooming bar (many are on the ground and some in my bedroom). I've searched for substitutions and haven't seen Luxardo cherry syrup from the jar of cherries as a sub. I understand that it's non alcoholic so maybe that's why, but in a cocktail like The Simon where it calls for only 1/4oz Heering I'd be interested to know what people who've tried Heering & Luxardo syrup think.
I agree with Dave, I think the maraschino cherry syrup would be an okay substitute, particularly at just 1/4 oz. I think adding a teaspoon of Cynar or Averna, if you have one of those, will balance out the sweetness while adding the herbal component that the syrup is missing.