If you go with the big bore kit,you will have to bore out the case 2 mm. OEM is 55 mm and the kit has 57 mm sleeves.
Then you will have the problem of feeding it. Forget about any kind of mpg. Horsepower is not free.
I am quoting from 2 people that have done the conversion. They also went with side draft manifold conversion.
One guy used a carb from a 500cc bike. Both had aftermarket exhausts. (drag pipe type).