I Did that conversion in the late 90’s to an ETZ, but instead of a re-bore I sourced a full 150 top end.
It did make quite a bit of difference.
From memory as well as a rebore or new cylinder barrel you will need a new piston and rings, a new gudgeon pin, possibly a different head, 150 inlet manifold, carb for a 150 or replace the jets in yours to make it 150 compatible and all assosciated gakets.
When I sold the bike I converted it back to 125, so potentialy somewhere in the garage I have the conversion parts you need – that being said they have been laid up for around 20 years!
Thanks for that, I think i will just go for standard rebuild at moment ,but if you come across parts in your garage I may be interested in them, I have a few 125 etz parts from a previous ownership that came with the bike.