A cylinder needs bored to the next size up when the cylinder/piston clearance gets worn and compression drops below spec, or it blows up for various reasons, take/send it to a shop and they measure/bore/hone/new piston to the next size that removes the wear or damage
You can overbore the stock cylinder like 8 times, the 240 cylinders only allow 3 over bores, and pistons can be scarce at times
once those bores are used up, you'll need a new cylinder or get that cylinder re-sleeved and start over
Porting/head mod, decent pipe chosen by where you want the power low/mid/top end, an aftermarket air filter w/ correct filter oil, and re-jet the carburetor for those new mods ..... and hang on
The link I provided you above for the machine work to cylinder/head and new piston, pricing is on the website
Here, eBay or rocky mt. atv for the pipe, filter and carb jets, we will guide you on jetting once you choose a pipe.