This is indeed the art of enterprise sales. 🙂
Marc Benioff and his team were legendary at this. Many seasoned enterprise sales execs are good at it, too.
The key is simply to acknowledge the gap — and commit to building what the customer needs within X months.
Enterprise customers are used to this. Sometimes, they will bite and agree. Other times, they’ll need more proof.
But it’s worth a shot if (x) you fill 90%+ of their need as is and (y) they believe you to be the overall vendor of choice.