In Missouri, we have to have the Highway Patrol sign off on the conversion, and even then, it has to be an engine that was originally available in that car. The Dept of Natural Resources administers the OBD testing mandates, but the MOHP oversees it. So we cannot put a TDI in a 2001 Passat for instance, without jumping through some hoops. But since they *did* sell a diesel in a 2004-5 Passat, and it is *mostly* the same car, they may let it go. The DMV will change the fuel type on the title, and when we (here at our shop, decentralized testing) put the info into our (the state's) system, we have to manually change the fuel type, so that the system looks for diesel OBD stuff instead of gasser stuff. I have not done a changed car inspection in a while, though, so it may have changed.
Thing is, if you live in an area that gasoline cars get OBD testing, but diesels do NOT, then bunches of people would be pulling the diesel card just to get out of the testing... so they HAVE to have some way of knowing.
The county in MO that I live in, Franklin, did not have any emissions testing on anything at all until around 2000, and they started with just the old BAR90, just for gas cars. Diesels were exempt. I actually had a heck of a time convincing the broads at the DMV that first go around that my 1991 Jetta, my 1998 Jetta, and the 1982 Rabbit pickup I had at the time, we in fact not only diesels, but they left the factory as such. I guess I must've had the only VAG diesels in the county.
So I ended up bringing in the titles, as well as pictures of the fuel filler doors of all the cars, and telling them that the next step was having them come out and lick the tailpipes. I was pretty pissed. The DUMB thing is, ANY diesel will pass a BAR90 test with flying colors... it may even have a hard time getting enough exhaust to test (sample dilution failure), so I was just about to drive them to the test stations the idiot state morons set up for a brief time (you wanna talk about a clusterfudge of EPIC proportions...
).
Eventually we got it settled. Then after a couple years, the BAR90 went away, the IM240 nonsense in STL Co and others went away, and we went back to the decentralized system and just a simple OBD test and just on 1996+ gassers/1997+ diesels and only with GVWR below 8500. We've thankfully been that way for a while now, and it may go away altogether.
So I would ask whomever your governing body in your state is, and see what they say.