I wouldn't sue over a usurious note without verifying the consequence with a local lawyer. At a minimum, there appears to be a statute imposing a 7% per annum penalty, Iowa Code Sec. 535.5.
You seem to be headed toward getting a default judgment, so one thing you can consider is making a claim for the money without mentioning the note, and perhaps also for interest at the maximum permissible rate. Let him bring up the note, if he defends against the claim.

