surprisingly (or not) , this is rather difficult to do in California. There is a referee that has to validate that the engine swap was completed successfully and without compromising the emissions equipment of the swapped engine. Though not every ref judges on the same scale. There have been reports of some requiring that the gas tank from the donor car be swapped as well (very difficult since the tank usually fills a space in original car that is unlikely to match with the recipient).

I'd like to see a more thorough emissions test be implemented (i.e. more than testing at two speeds, enough to gauge all operating conditions) so that it could be more objective in its process. This would also have the benefit of allowing a way to bypass the restrictive modification exception system.