i dont think youll get a whole lot different performance moving from a 14mm bing to a 15mm. carb size is not quite as important as other aspects...you can have a fast bike with a small carb easy.
dont overmix the gas. that will not help your performance. 50:1 is a safe starting point, adding more oil will only bog you down.
top gasket isnt even needed. i never ran one on my bing or my dellortos and it made absolutely no difference.
the engines CC is determined by the size of the bore in the cylinder. lets say you have your stock 50cc cylinder, when you look down it, its this size visually: o ...but you look down an 80cc cylinder and it will look like this: O ...horrible example but it kind of explains it.
disregard the cc rating on a head, those treats 50cc heads are just designed to be used with a 50cc engine, but since your piston never touches the head (ideally..) it makes no difference.