RHD: isnt the problem RHD isnt per se illegal in the USA.
However a Skyline JDM car needs a lot of mods to bring it EPA/DOT compliant, this will run between 15-20 k depending on the company.
I can imagine someone registering a R32 as a S13, but that is highly illegal, though the chances of getting caught are slim in some states.
There is a cottage industry in place that legalizez JDM cars especially Skylines for good $$.
The Skyline forum also has posts on this subject of legalization.
Fred..
Edit for typos.