Check out this link
In just 12 hours I broke my new 300td
I PM'd Dakotawind who replaced just the radiator top and it was working like new. The only difficulty is that when I called them I found out that the supplier sells wholesale only, so either make yourself out to be a radiator shop when you call or get a local shop to buy it for you...
I didn't replace the top on mine, I put a new one in.
Good luck!