Replace ivygen leaf cards with custom leaves is there anyway to replace the leaf cards that ivygen generates with custom modeled and textured leaves (a group) but keeping the original orientations of the leaf cards from ivygen?
Most often you don't want to do that: think about the amount of polygons that would be. (You could kinda get away with it if you work with Ivygen at a small scale. Good luck with doing it with a tree in the Sapling add-on...)

The smart way as long as the camera is far enough to get away with it is to alpha map the leaves. (I believe they are UV unwrapped by default, I could be mistaken.) This reduces the poly but gives you a greater amount of detail.

If you want an example of this technique out in the wild, have a look at this piece from blenderartists: http://blenderartists.org/forum/showthread.php?382855-Fairytale-Village

If the camera is too close for a pure alpha map, you may want to use dupliobject (and specifically duplifaces) to give it some depth. These approaches can be mixed: the exact outline of the leaf can be done using alpha maps to save on polygons. For duplifaces, see here: You could use dupliobjects. See here: http://wiki.blender.org/index.php/Doc:2.4/Manual/Modeling/Objects/Duplication/DupliFaces