It is a good idea to get a good Bill of Sale for your purchase. Make that a document to cover all of the information needed for your state's requirements on the vehicle.
You will sometimes need a "Title" as a document from the seller to give to the DMV of your state. If so then you will get a replacement title for the one given to the state at registration time. It is usually mailed within a few weeks.
Check with the DMV of your state to be sure.
This form of receipt or "Bill of Sale" should describe the details of what you are buying.
As there are various types of Off Road Vehicles such as ski-mobiles, ATV's, Dune Buggies, Dirt Bikes, Rock Crawlers and even regular vehicles used with sometimes an adaption such as a Jeep etc. There are tracked vehicles as well.
The requirements for buying and registering them vary so check with your states DMV for the details.
If you buy an off road vehicle with a simple sales slip or a hand written note rather than a bill of sale that is without all of the information needed, can become a problem in the future.
Make it clear that both parties understand the details of the agreement, use a good, descriptive bill of sale. There might be exceptions but, do not forget to get a title for the vehicle signed over to you.