Bullpup rifles are not prohibited if the bullpup stock is an integral and required part of the operation of the rifle.
So, if a rifle can work in a non-bullpup stock, then a bullpup stock for that rifle is prohibited.
Example:
Type-97 is not prohibited (thus far) because it was designed as a bullpup and that's the only way it works. Non restricted thus far.
There are bullpup stocks available for the sks, but they are not _required_ to make the sks function. SKS can operate in a "regular stock", therefore, sks bullpup stocks are prohibited devices.
Stupid? Oh #### ya.
The best available is probably the Tavor, or the type 97.