Well - if its titled as one vehicle and looks like another, I think the law will have a problem with that. If an officer is behind you, pulls up your registration, and the vehicle clearly does not match the registration, he'll likely want to pull over and have a chat.
Here in MA we have similar laws, except all vehicle that are older than a 1984 are excempt from emissions standards. Therefore I can run pretty much any engine in my 1983 truck, as loong as the truck passes a safety inspection. There are some legal engine/drivetrain swaps, but I'm not familar with how that works.
2012 Chevy 2500HD ECSB, 6.0L gas, auto, 4x4, LT
2003 F450, 7.3L PSD, 6 spd stick, 2wd, dump truck
1983 GMC K2500 pickup - working on a Cummins 5.9L swap
1987 GMC K3500 Dump, 6.2L diesel, TH400 auto - project plow truck