If you are going to resort to building from scratch (the kit plans won't help a lot, because they won't help you make the specialized molded and vacuformed parts, or to get the out-of-production tubing) you can find dimensioned drawings of the Mercury-Atlas in "Rockets of the World."
(disclaimer: I'm the author and publisher, and get anywhere from 60% to 100% of the money depending on where you buy it)
Peter Alway
Saturn Press PO Box 3709 Ann Arbor, MI 48106-3709