The Jinma Chinese loaders will fit the 504, but I'm not sure of the specific model number for that loader. My 304 and the 354 take the ZL30 so theremay be a ZL50 for the 504.
Koyker makes a good loader to fit numerous Chinese tractors. You can give them a call and they can tell you which one would be right for you. Then there's the Coldwater loaders – Google them. Spirit loaders are made in the USA, Minnesota, I believe.
Affordable Tractor Sales, one of the advertisers and advisers here, sells the TomKat loader, made in South Korea, I think and highly rated. They should have one that would fit your 504. Or maybe I'm thinking of backhoes…call Tommy at Affordable and get his advice.