Should not be a problem. I bought a XR400 and then got Colorado OHV registration. I don't think they even needed to see the bill of sale. My next step is to title it.
From another forum:
You need form DR2686 "Certificate of Equipment Compliance".
This form states you need:
A completed DR2087 "Verification of VIN",
and one of the following:
"Manufactureres Statement of origin (MSO),
Bill of Sale (stating sold as off road)
Dealers invoice"
That's it, then you sign DR2686 and you get your title.
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From another site:
Dirt Bike Registration
Dirt bikes (or off-road bikes), on the other hand, are not considered motor vehicles by the state, but still need to be registered with Colorado Parks and Recreation. You can download and print the registration application, complete it, and mail it with the appropriate fee ($25.25) to:
* Colorado State Parks
* Registration Unit
* 13787 South Highway 85
* Littleton, CO 80125
You can also fill out an application in person at this address. It takes about two to four weeks to receive the Colorado Registration Certificate.
Making it Legal
Now for the tricky part: You can transform a dirt bike into a road-legal motorcycle recognized by the state of Colorado as a motor vehicle. With a bit of tinkering and upgrading (generally new tires, a horn, a new muffler, and proper lighting) the bike can be titled and put into the periodic registration process as a regular motor vehicle.
Of course, you will need to fill out a couple of forms. First is the Certificate of Equipment Compliance, which lists all the equipment needed for the dirt bike to become street legal. The second required form is the Verification of Vehicle Identification Number.
Along with these two forms, you will need proof of ownership for titling, which can entail a manufacturer's statement of origin (MSO), bill of sale, or dealer invoice.