Learn Spanish (Latin America): Ojalá tuvieras buena memoria para recordar ese momento. - If only you had a good memory to remember that moment.

Because this is an unreal or highly unlikely wish about the present. By using the imperfect subjunctive (tuvieras), the speaker is saying 'If only you had a good memory' (but implying that you actually don't). If they had used the present subjunctive (tengas), it would change the meaning to a hopeful, possible wish: 'I hope you have a good memory.'

Could I say Ojalá que tuvieras instead of just Ojalá tuvieras?

Yes, absolutely. With ojalá, adding que is entirely optional and doesn't change the meaning. You will hear native speakers use both forms interchangeably when expressing these kinds of wishes.

Why isn't there an una before buena memoria to match the English translation 'a good memory'?

In Spanish, it is very common to drop the indefinite article (un or una) after the verb tener when talking about a general trait or something you would normally only have one of. Saying tuvieras buena memoria sounds much more natural here than adding una.

Why do we use para before recordar?

We use the preposition para here because it expresses purpose, meaning 'in order to'. The phrase translates literally to 'If only you had good memory in order to remember that moment.'