Hi Dan,
Thanks for help it was very much appreciated.

I'm trying to get hold of a CBR150 crankshaft, which should fit straight in, replacing 125 crankshaft and also fit a new piston. I agree brass was probably not the best choice, but live and learn. The groove in the cylinder wall is quite a nasty one. I was surprise the engine was still running considering what I found when I pulled it apart.