I am sorry for your bad luck there- I have a Rossi-45 and 60 and the 45 ran perfect out of the box....
I cannot imagine how LOUD it must have been after that- did you get her down safely?
An unbalanced prop wouldn't do that (at least I wouldn't think so..) sounds like bad material in the pipe itself-
Send it back- and let us know what happens...
If you wanna sell the engine cheap let me know-