As it is filled and levelled by the fill hole I am not sure how you'd over fill it? Obviously done on the level, not jacked up at one end or side only!
Stick with the 1.2 litres it'll be level with the fill hole bottom lip within 0.5 mm depending on temperatures of oil and box.
Maybe the 1.7 is a classic Morgan Typo! and oil changes are supposed to be messy
