If your piston is bad, I would look closely at the cylinder wall. Replacing both piston and cylinder is a common venture. They are available in combination from a lot of sources. Likely the Brozz piston/rings/cylinder will be a bolt-on, see here:
https://shop.brozz250.com/eshopprod_...r_Assembly.htm
Note the image(make it bigger) and you will see the cylinder head, which should confirm the part is correct.... I think. Mebbe yours is smaller bore??
You may look here:
https://www.aliexpress.com/item/32769547579.html?spm=a2g0o.productlist.0.0.6f86170 cLwQQAU&algo_pvid=022f276f-6045-41c3-8f0b-93cd62395cc9&algo_exp_id=022f276f-6045-41c3-8f0b-93cd62395cc9-31&pdp_ext_f=%7B"sku_id"%3A"62282761417"%7D
Air-cooled piston/rings/wristpin/cylinder/gasket for CG250