Replacement - Any Ek ARB should fit, so in terms of replacement its down to what ever you can get for the cheapest. Sounds like you've a picky MoT tester, as I can't see how corroded structurally they can get unless its seriously old. Take it off blast it and powder coat it if you're not too bothered about upgrading.
Upgrade - Anything bigger than your standard 15mm one. 22mm would be good of an Ek9 but they are often over priced. See if you can find a 22mm white line ARB and replace your droplinks while your at it. Should find it feels a lot more planted and flat around the bends, with a lot less under steer.
Hope that answers your question.