This is an extremely hypothetical question, based on a worst-case scenario. The treaty does not contemplate failure, and it does not contemplate returning to the Indian Act. A failure of this magnitude would be something that governments and financial institutions would want to avoid. Any remedies for such a situation would depend on the specific circumstances at that time. The International Monetary Fund is not applicable to First Nations, but there will likely be other remedies available if this happens.
There is currently no fund to “bail out” Indian Bands if they go bankrupt.