Maybe I’m remembering wrong, but I do not believe there is a statue of limitations. In fact, I’m pretty sure that I recall that once he has been convicted, any two people who were present can later go to another bais din, testify that the defendant was convicted of a capital crime (along the normal rules of testimony) and the new bais din can execute him. The original witnesses need not be present and the original testimony need not be rehashed.
The Wolf