You might want to try using the Oblique plugin, which lets you skew objects. http://forums.getpaint.net/index.php?/topic/2524-oblique-effect-plugin-ymd100714/
I think true isometric angle is 30 degrees, so when you have the plugin, follow these steps:
Duplicate the layer.
Use the Oblique plugin to skew the fence. (You may need to do this on a larger canvas, since if the image goes off the edge it will wrap around.)
The numbers you're looking for probably have something to do with 30 degrees, mess around and see if you can get it perfect.
Let me know if you need any other help!