How do you handle repayments on a previously defaulted loan, both for accounting and 5500 purposes? defaulted loan was treated as a deemed distribution, participant is still employed, and there is no distributable event.

Payments equal to the amount that was treated as taxable will create basis in the participant's account. Otherwise, loan payments are treated the same as if no tax event. They are not contributions, if that is what you are asking. The loan did not go away for accounting purposes.

What type of plan is it? Small (Schedule I) or Large (Schedule H)?

For a Schedule H plan, the deemed amount should have been reported on Part II, Line g in the year it was deemed.
If the participant has started repaying, then the deemed amount is deducted from the amount of deemed loans in the year repayments started. It is my understanding that this amount could be negative.

For Schedule I, same proceedures (even the same line).

For accounting purposes, I think the deemed amounts are still considered assets and should never have been removed from the financial statement, however, I am not an accountant.

Any other thoughts?

That is correct. The financials would have never taken the loans off.