Hi Lyllen.
This is quiet a complex thing, but it`s no point just to change either the bore or stroke without any other modifications.
With the 75 stroke crank and standard pistons you`ll get 699cc and you could get away with just an adapter plate under the barrels.
If you change both at the same time you`ll obviously get more cc. If you go too far with the bore you`ll have to open the block up and also get it strengthened if you use 85mm pistons and barrels.
As I said there is loads of variations, depending on what you are after.
Even with some bigger valves, sporty cam shaft a better carb lighter flywheel and some little modifications you can get a much more lively engine.