To transfer a dirt bike all that is required is a bill of sale listing the Make, Displacement and serial number, the name and address of the seller and name and address of the purchaser, and the date and amount of the sale.
Most states prefer that you use the form printed on the title - but because they are not registered for on road use there is no other follow-up.
All street legal vehicles - including dirt capable "On/Off road" motorcycles require ownership be transferred by use of the form on the state issued title, and that the transfer be registered by the Secretary of State or DMV.