Is there a way to attack a federal conspiracy indictment for lack of evidence

The only way to attack a federal indictment that charges a conspiracy count based on a lack of evidence is to try the case. "Back in the day," the Government had to prove the existence of a conspiracy for other evidentiary purposes (not relevant to your question, so I won't digress), and if the conspiracy count was weak, that pretrial hearing would usually cause the Government to re-evaluate its case. However, that requirement has all but been completely eviscerated.

As far as my knowledge there is no such motion, you would have to attack this evidence at trial.