This is my third attempt to reply to your question. I don't know where the other replies went to , but I'm trying again.
First off, I can't refer you to a commercial supplier of Leavitt's plans. And by the way, those plans appeared in Mechanix Illustrated magazine. I never saw the 1947 MI issue but while in high school (the mid 1960's) I came across a MI special edition from the early 1950's; a compilation of modeling articles from preceding years. Leavitt's article was in that special edition along with a notice to send $3.00 for plan #428. I did - cash - and by golly, they sent me the plans. I began work on the model soon after and quickly realized I was in over my head and put the project off. I never lost interest in the Lexington though and was content to wait until my modeling skills matured.
In 1978, I came across Charles Davis' The Built-Up Ship Model. Davis' book was first published in 1933, preceding Leavitt's article by quite a margin. As time went on, I acquired other plans for the Lexington; George Parker's, published in 1976; Rolf Hoeckel's, published in 1985 and in the early 2000's, Clay Feldaman's Lexington practicum.
In 1994, I acquired the Nautical Research Guild's Ship Modelers Shop Notes. Beginning on page one is Howard Chapelle's article "The Ship Model That Should Not Be Built". The Lexington is among those ships he admonished modelers to avoid and his article speaks to your question of validity. No historical plans for the Lexington exist.
I was shocked and confused. At that point in time I had four sets of plans for the Lexington! (A nagging question remained though - above the water-line, Parker's plans did not resemble the others). I was in correspondence with Harold Hahn at that time over questions about another model and asked him about the veracity of the various Lexington plans. Here is his reply: "Most authorities believe that the popular design for the Lexington that Charles Davis dreamed up would be more correct for a ship built in the early 1800's. Actually, I have a plan for the Lexington that Davis had drawn earlier which is quite different. The early references you have including Clyde Leavitt were simply copies of Davis' design. That is also true of the Crabtree model...I don't know of any authentic plans for a Continental Navy brig." I will add that Feldman's version conforms to Davis' design.
Faced with the facts from Chapelle and Hahn, I decided to shelve my Lexington project.
If, after all this you are still interested in building the Lexington, I would be glad to send you full-size copies of Leavitt's plans taken from my originals. The copies show wear present on the originals but are completely usable.
As far as I know, all that is known about the Lexington is that is was a commercial vessel, the Black Duck, converted for naval use. Its beam and length-on-deck are known and that it sank in the Caribbean.
Charles Green, Boise, ID