I'd rather just buy a 150cc motor and go from there. It wont cost much more and its a better motor overall. Also any dumbass can install one of these motors with ease.
A Lifan 150 will cost you $500 to $550 delivered ... You can buy a TB 57 mm ceramic bore kit (142 cc) for your 125 for around $200 from most dealers but you can sometimes "win' them off ebay for $99 + $10 postage ...
What you're up against ATM is the fact that the weak AUD will cause prices for imported goods to be dearer that they used to be ...
As I-Thump has stated ... you might be better off selling your 125 (actually 120 cc) off to someone for $150 OR as much as you can get for it ... then investing in a 150 or even a YX160 ...
You can buy 140's for around $365 + postage but 150's are much better ...