Re: Tank advice
It kind of depends on how professional you want it to look.
If you want it to look amazingly, then it's going to take a lot of work, possibly some ordered parts, and maybe tools and skills you don't have. But if you don't mind it being somewhat of a "hack job" then you could just take anything water tight and with a cap (another fuel tank obviously being preferable) and just get a Y-connector to connect both tanks to the carb. You could (and should) have a shutoff valve for the secondary tank too.
Anyway, that's just my thought, you'll probably want to wait until a more experienced builder says something.