Re: gas tank
Check your local yellow pages. Most large cities have at least one motorcycle salvage yard. Here in Denver we have Steel's M/C Salvage. Hundreds of bikes to look at and scrounge parts from.
One suggestion would be a tank from an early Yamaha MX 80. I have two Early Harley Baja tanks that are slim and have a great shape, adaptable to a bicycle frame.
Good luck. Let us know what you come up with.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"