Don't know if this will help, but I live in NH and have registered several old bikes with only a hand written bill of sale. Go to your town clerk or to the DMV website and get Form TDMV 19A Verification Of Vehicle Identification. Take the bike or just the frame along with the form to a motorcycle inspection station or to your local LEO. They will verify the VIN and complete the form. Then take the completed form and the bill of sale to your town clerk or DMV and get it registered.
NH doesn't require or issue titles for older bikes but I'm not sure what the age cutoff is. The bikes I registered were 1971, 1977 and 1978. Also, two were street bikes and the 71 was a Yamaha enduro. Not sure how this will work with your bike since it is newer and a dirt bike. You might start with just a phone call to your town clerk and ask some questions. Good luck.