Seeing that you're in NZ ,I'd look on that Trade me site as a complete mower is $50.
If you do a rebore it could cost $150 to $200.
eBay have pistons etc.
Probably have nothing to loose if you try honing the bore ,if you don't have a hone I'm sure a Motorbike wreckers etc could hone the bore at a cheap price but if it was me I'd just use this one for parts and buy another mower .
