Would love some tips on how to turn down a project while still leaving the door open for future work with a client.

Background - This potential client saw an annual report I created for a nonprofit and loved the concept (front page was sort of an infographic / illustrated story). They want to do something very similar...however, the outline they sent me doesn't lend itself to the same treatment as the work for the other client. Basically, they are trying to squeeze a square peg into a round hole, just because they like the look of the square peg.

I initially told them that I was so busy that it would be at least a month before I could even begin on their project (which is true!), thinking that would be the end of it. Apparently, they are so enamored with the work I did for the nonprofit that they are willing to wait and want me to put together a proposal...which is certainly flattering, but I honestly have no idea how to turn their outline (which has an overwhelming amount of detail for what would be a one page marketing handout) into a finished product. I'm ready to decline the job and suggest they simplify their concept, but would love to hear some advice on how to do that in ways that at least leave the door open for future work.