One can easily look at the blue prints for the U.S. Mark-III nuclear weapon (FatMan) as it is entire declassified except for the nuclear trigger. Much of the Mark-I bomb (Littleboy) design is also declassified. The truth is a nuclear bomb using HEU is a very simple design and would not be difficult to build. The challenge would be getting enough HEU, forming it into the necessary shapes, and then getting the high explosive shaped charged to detonate properly.
The sad issue is even if they are somewhat off on the charge and the shape, they device would still go "atomic" although the yield would be less......depending on the size, quality and amount of fissionable material a Hiroshima size yield could be expected.