All of those "assualt rifle" laws I know of have the primary defininition of such as "a semi-automatic rifle". If it isn't semi-auto, it isn't an "assault rifle." I should also be easy to permanently modify the SKS and AK to be straight pull bolt rifles with a return spring, just by disabling the gas system and welding on a longer bolt handle.
Modifying the AR-15 in that way would be a bit more difficult.
Another modification to comply with the law would be to (semi) permanently attach the magazine, so it is not detachable without tools. That negates the second part of the "assault rifle" definition, a "detachable magazine". That has been done.
A third way, which would require receiver redesign, would be to modify the AR-15 to have a fixed magazine, but a stripper clip guide on the side (a la Johnson) to load it with 10 round clips. Again, no detachable magazine, no "assault rifle".
Jim