Re: US to scrap nuclear deal if India tests weapons - State Dept
I always thought the tough part of nuclear energy is the engineering and not the underlying science which is fairly well know to people who have completed a decent undergrad degree. Safety, backups, training, materials & qa are incredibly complex. there is less scope for modularisation than one imagines.
In other words there is no need to reverse engineer because there's nothing to discover.
The biggest challenge if US balks at it will be for India to get materials - not just the visible uranium derivatives but a bunch of other things. They can but all that becomes easier and cheaper to procure with US help (eg: read about Australia).
Pakistan's program was brought to dead halt several years ago. Not clear whether even necessary peiodical refresh was done, though signs are that they were.