I bought that kit (Otom "300cc" piston/cylinder).
Attachment 30267
Attachment 30268
Oh it is a NICE kit. Complete, and the machining and parts look excellent. All gaskets, orings, gudgeon pin/circlamps, piston rings. Very nice kit.
But I was dismayed when I found out it was just a 74mm bore. YUP, that increases displacement by 14cc (249.99 to 264.07). I wouldn't call that a 300cc kit, even though 300cc is cast into the cylinder (see it in the photo). At least the advert you found admits it is a 264cc kit!
For me, it's NOT worth it unless I also add the cam, port the head and maybe get a slightly larger diameter carb. All together, it would probably make a big difference (maybe up to 24hp). The most noticable difference might be in the midrange torque feel, and off the start torque would probably feel spunkier and more willing.
So my kit will sit on the shelf until I have time to really work on it.
I also got a new head and was willing to do it, but it did not come with the valves and cam (listing showed the valves/cam included). Disappointment.