You can use a 4x4 pump in an RS500 if you can find an original new one rather than these re manufactured ones with known problems.
I have a brand new original RS500 one in my spares, they do come up for sale every now and again as old stock items. I wont part with it sorry just in case I need one myself in the future.
A relief valve spring and cap kit is only £10 or so and simple to change but you should not need to do that really on a brand new item, but maybe its the only way you are going to get one that works, especially given how far away you are.