Agile is about doing things which provide value to the customer or product owner. 

You have an API which fulfills its purpose in its current form, but it is not really REST compliant. The only reason to extend effort to make it REST compliant seem to be that you at one point decided that it was supposed to be REST. That line of thinking is not agile.

REST can of course be perfectly fine in an agile project if you actually need it. But it seems you don't.