3459075 ball diff bushing. there may be more numbers but this is from the 414x manual. I know Tamiya America does not show they have it (just checked), but they may have it under another number, if you call and talk to customer service. I think Tamiya released it under another number a few months ago.
As for use of the alloy pulley. I have seen them come apart. the pulley is actually two pieces (melted) together. I have melted a couple rears when I had the rear diff too loose (the balls actually ended up between holes). But as stated earlier the best application is the dusty / dirty tracks. If your on a well groomed track and looking for the best performance then go with the plastic pulleys front and rear. Lighter is better, especially when it comes to rotating parts.
Hope this helps