Urgh. my carb main fuel lines are perished thanks to modern gas - and replacements are no longer available - anyone got any thoughts on what I can use instead?

 

Take a sample to your local auto parts store and ask them would be my advice.

 

I recommend just buying a couple feet of FUEL Hose Line in the size diameter you need.
Also, get a couple of junction connectors/elbows, plus clamps. This will allow you to make neat bends, without kinking the hose on a sharp bend replacing the factory ones.