Assuming that you are not making any money with this project at the moment then you don't have a lot to lose - except perhaps more time on a project that ultimately goes nowhere.

With a low risk then you can offer a new founder up to a third of the business. The challenge you should set is to "find the customers". This may well involve at least a partial "pivot" - and that is where the extra work comes in.

You don't have to incorporate and formalise the arrangement until the new revenue stream is located and at least partially proven (you can just write and sign some sort of informal agreement). Once you have a clear view of your customers, what they want from the product and how much they are willing to pay then you have a business and a proven marketing founder.