can I use knox gelatin instead of agar in the Eggless Lemon Curd recipe?

They set up exactly the same.

If you're using the eggless recipe to make the dish vegan, keep in mind that most gelatin is not vegan. Knox gelatin is made from the collagen found in the "bones, skin and connective tissue of animals" (Source: http://www.knoxgelatine.com/basics.htm). Agar is an algae-based product that is often used as a vegetarian alternative to conventional gelatin.

yes.