Welcome to Iboats,
Can't use a remote tank besides internal 1.75 Ltr one, unless you hang one higher than engine and or on top engine and connect a fuel hose direct to carb. If possible doing so will need to install a fuel filter and a fuel valve between extra tank and carb, bear in mind that 3.5 Ob's doesn't have a fuel pump, gravity does all the fuel intake to carb. Post whatever crazy idea you end with, love innovative crazy ideas myself.
Happy Boating