This is common. There are a continuum of choices.
On the one hand as a consultant your job is to advise and recommend, so you can write a report with your recommendations, including advising raising more cash on the basis of the recommendations and leave it to the inventor to either do that and engage you for implementing the recommendations. This is the most passive approach and conserves your resources.
On the other hand, in the most active role you act as entrepreneur: you find suppliers or contractors who willl work for deferred cash, royalties or equity, you find distributors who will advance cash or invest, you find lenders and investors who will give the company working capital and to cover your fee. You might even take your fee inequity or a percent of new sales.You wrap up all the proposed contracts with a bow, give it to the inventor and say my recommendation is that you sign these contracts that will bring about the enhancements I have proposed, and finance the work and my fee. If the inventor accepts them you get your fee. If they don't you go unpaid.
And since this is a continuum there is all the possible solutions in between.