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On a technicality you "can't" register your buddy's bike for him. In NH person doing the reg is supposed to check ID of the person registering to match that of the registration. Not saying they all do but that is the rule in NH.
If your buddy and you are ok with your name being on the reg for his bike than you can get a sticker for his bike with your name. All that is needed in NH is the VIN, some money and your ID to register an OHV/Dirtbike.
I'm still working on being ready to go that weekend. Bottom end of motor is together for the most part, waiting on a few small parts I didn't know I needed to finish it up. Rest of the work shouldn't take too long once I make a day to do it. I'll be playing it by ear.