Bingo! If they can fake SID chips, disks are a cinch and cartridges only slightly harder. At least those last two probably work.
You can buy all the stuff you need to easily make C64 cartridges from ebay, it's usually the guys selling the fake cartridges who'll sell the stuff to make your own cartridges...
PS: I should know, I bought the bits'n'bobs to give it a try and it work perfectly...