Welcome along. They are pretty bulletproof engines, I've owned mine for over six years and never had a problem, a tip would be check your oil level regularly. That's what kills most 350's , if the light comes on it's usually too late
I've got a schwinn rocket 3 too. Think I've had it about 12 years, uprated wheels, forks, disc brakes, fox shock, gears etc. Quite a nice bit of kit in its day.
I don't think there are any aftermarket options (I've never seen any) so it would be second hand or new from Nissan. I'd clean yours up if I were you. The seal may not of gone on the headlight, sometimes they do just mist up and it's more to do with air flow than a seal. Aston Martin retrofit a breather tube to the back of the light to cure their misting problem!
There was a company doing headlight painting and demon eye/ halo fitting on here a while ago, maybe they could clean up your lights and refinish them?