You're going to do a lot of that content later on, so if you do it now, you'll just sit in class being bored.
At this stage, it's probably more useful to make sure your foundations in number, algebra, geometry, trigonometry are strong. Parts of the 1/2 textbook that are good for this are:
Chapters 1 and 5 on linear and quadratic equations.
The first half of Chapter 2, on arithmetic techniques with real and rational numbers.
Chapter 9 on geometry of triangles and quadrilaterals. Possibly Chapters 10 on circle geometry, if you did that in Year 10 as well.
Chapter 13 on trigonometry.
Apart from basic skills, make sure you practice working through non-routine problems and recognising when and how to apply techniques - there's far less cookie-cutter maths in Specialist compared to Methods. So try the extended response questions in the chapter reviews.