Should somebody in year 10. 5.3 whose getting 62% for their tests do extension 1 maths? If I did do extension 1 maths I would probably get a private tutor. Also last year I was in 5.2 and got 89% for my yearly.
Considering that you still have like 3 exams left in Year 10 I would honestly just recommend understanding all of the year 10 content first, stuff life Algebra (Factorising Quadratics etc.) and Trig (Sine and Cosine Rules, Bearings etc.) often come up a lot in 2 Unit (Advanced Maths) and X1 so get really good with those. If you can get to a point where you're getting around 80%+ on your 5.3 exams then I would seriously recommend picking MX1 up in Year 11, as always though ask the teachers at your school what they're opinion is on your capability and whatnot.
NOTE: There may also be an entrance exam into X1 for any Year 10 students looking to take it on in Year 11 - it may not be completely left up to your performance in your Half Yearly's and Yearly's.