I got the game Demon Castle Legends (Castlevania III: Dracula's Curse) in my mail (Got it from eBay, seller from US). I tried to insert the cartridge in my Super Joy (got mine back in 2005), but the tabs in the cartridge slot blocked my game from inserting it. :(
*sigh* I have to wait for the Retron 5 to come out, because it has the Famicom cartridge slot. OR buy myself a needlenose pliers from a local hardware store...
UPDATE: I got that knife from my local auto store, and cut out the tabs off the cartridge slot of my Super Joy, and after I inserted the game Demon Castle Legends (Castlevania III: Dracula's Curse), the game doesn't come on. Which means that this game is not compatible with the Super Joy or any of its variations...
Or get a real Famicom since you got an expensive game with excellent music anyway.
Does the Super Joy take any other cartridges?
I think on my super joy I had to cut some plastic around the cartridge connector for a game to fit properly.
Do what P says and get a actual famicom,I would recommend a av famicom.
And Im not so impressed with the Retron 5 as all it does is dump your game to memory then you get to play it.