I found the definitions in Title 45, chapter 1: 49-123(n)-(p).
Idaho 49-123(n) wrote:
(n) Replica vehicle. A vehicle made to replicate any passenger car or truck previously manufactured, using metal, fiberglass or other composite materials. Replica vehicles must look like the original vehicle being replicated but may use a more modern drive train. At a minimum, replica vehicles shall meet the same federal motor vehicle safety and emission standards in effect for the year and type of vehicle being replicated.
Idaho 49-123(p) wrote:
(p) Specially constructed vehicle. Every vehicle of a type required to be registered not originally constructed under a distinctive name, make, model or type by a generally recognized manufacturer of vehicles and not materially altered from its original construction and cannot be visually identified as a vehicle produced by a particular manufacturer. This includes:
(i) A vehicle that has been structurally modified so that it does not have the same appearance as a similar vehicle from the same manufacturer; or
(ii) A vehicle that has been constructed entirely from homemade parts and materials not obtained from other vehicles; or
(iii) A vehicle that has been constructed by using major component parts from one (1) or more manufactured vehicles and cannot be identified as a specific make or model; or
(iv) A vehicle constructed by the use of a custom kit that cannot be visually identified as a specific make or model. All specially constructed vehicles of a type required to be registered shall be certified by the owner to meet all applicable federal motor vehicle safety standards in effect at the time construction is completed, and all requirements of chapter 9, title 49, Idaho Code.
I believe these definitions come somewhat from SEMA laws enacted in 2008.
If you're building a Lotus Seven replica, and it looks recognizably as a Lotus Seven, then I would say that the replica vehicle definition fits. Also, I'd be hard pressed to argue that your vehicle would comply with (p)(i)-(iv), at all. I would take a picture of what a Lotus Seven looks like, and perhaps some pictures of finished book builds, and see if her opinion changes. Otherwise, just break out the statute book and start nitpicking the laws.