Having an old nuke would only help a good engineer reverse engineer the assembly of a new device.
...and the assembly isn't the major technical hurdle. The fabrication of fissionable material into a perfectly shaped pit is a major technical hurdle, on the other hand. But just seeing the remains of what used to be a weapons-grade pit won't help much. Obtaining enough fissionable material is yet another major technical hurdle. Maintenance is yet another.
But just seeing the components of a working (or once-working) device won't really tell you how to construct a new one. Fissionable material has a radiation field that must be respected in construction, design, movement, and assembly. Fissionable material also releases heat that must be taken into account. FM can also go critical if your components ever come too close to each other, among other "pit" falls to building such a device. FM is among the most brittle of all metals, as well as extraordinarily susceptable to rusting into plutonium or uranium oxide. So machining such metals into precise nuclear tolerances is not for amatuers.