Aliexpress.com has many kinds of parts (and other things)... from cylinder heads to carburators for GY6... even a cylinder assay. It's a chinese company but they ship. Watch the shipping costs (some things are free shipping but not all) and the minimum number to purchase... sometimes you have to buy 10 of an item. Just go to their website and do a search for "GY6 motor" and see what they carry.
Did you need the whole engine? What is broken? Did you want a larger cylinder? Is the cylinder assay broken? Is the carburator any good? The GY6 engine is extremely common in China and the Phillipinnes but there are a few places stateside that carry parts... you local scooter shop for one.
Good luck.