You can get great power with just porting. Thing is, you already have a bad crank. If you have them money to get porting done and a head, why not get a stroker crank. You can replace the intakes and carbs later if you want to and figure out filtration set up.
4mm stroker add 4mm to the stroke distance of the rods. 2mm higher and 2mm lower. The pistons will actually stick out 2mm above stock cylinders. That's why you need a modified head/domes or a spacer plate.
Depends on the money you want to spend. Like said above you are probably looking at $1000 to $1700 to make it right. If you just get it bored, new stock head, piston kit, gasket, seals, and crank you can proabably get away with $700-$800.