This reply I saw is for Texas only since each state is different:

If the vehicle has a certificate of origin it can be titled as a kit car (if it came with no engine). All that is needed is a weight certificate, state inspection sticker, proof of insurance, the title from the producer (c.o.o.), and bill of sale for components used. (engine and trans). If no certificate of origin is available (no vin number) then a trip to the Texas D.O.T. will have to take place and a police officer will have to inspect and approve it, then a vin # will be assigned and stamped on the body or chassis and the above steps can then be done. If there is a vin number but no certificate of origin a pencil tracing of the vin number must be presented.

Some counties require emission testing and the ones that don't require the emission components that came with the engine be in place. If a Certificate of origin can be had by all means do it. If you can eliminate the trip to the Texas D.O.T it will shorten the process tremendously. I made three trips to the license place and one to the Texas DOT to straighten all this out. These rules can be bent to various degrees and it will depend on who helps you in the local license office as to how much you can get away with. It could come back to bite you in the future. One thing to concider: A kit with no engine assembled as a kit car requires no sales tax be paid. A complete kit or car would require sales tax be paid. Sales tax on a 25k kit would be 1700.00.