First outing with my son and the Durga ended with two broken diffs

[built2scale]

Well, as my title stated, our first bash in a parking didn't end on a good note. Found both front and rear diff pulleys torn apart.

Much like this person's pics,
http://www.oople.com/forums/showthread.php?t=20313

So what should I do now? I've read bits here and there about using a 501x diff but I don't know what parts I would need. Is there a complete set to buy or bits and pieces?

Everything else looks good so if I keep with the Durga instructions, it looks like I just need the two 37T pulleys. What do I need if I want to upgrade to the 501x diff? Just slap in a 501x pulley or is it more complex than that?