Hmmm... Just found this page:
Colorado emissions program
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> Emission testing of gas and diesel powered vehicles is required when registering, reregistering or selling vehicles in the following Colorado counties known as the Enhanced Area: Adams, Arapahoe, Boulder, Denver, Douglas, El Paso, and
Jefferson. Testing is also required in Weld, Larimer and Pitkin Counties, known as the Basic Area. <HR></BLOCKQUOTE>
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> Vehicles registered and licensed in the categories of: horseless carriage, street rod, farm, and collector series are exempt. <HR></BLOCKQUOTE>
I don't know my Colorado counties, but you may be exempt on that basis. Otherwise, I'd look into what the requiremetns are to register it as a street rod or collector series.