Is it possible to do a Master's in Mathematics after a Bachelor's in Economics? I'm currently an Economics student in my second year (only just begun).I've come to realise that I'd like to study mathematics further (pure or applied) since I'm missing the subject ever since school ended. The maths in undergraduate Economics isn't too challenging so I'm not enjoying it too much.

There isn't too much flexibility in my university (I live in India and the education system is rigid) so I do not have an option to double major in Economics and Maths. Also, I will have two allied courses of Linear Algebra and Calculus as an add-on. 

I've been a fairly good student in maths so far and I've already started to take on some of the undergraduate maths courses on my own. Would a good maths GRE score be sufficient to apply to a good university in the US? Could I also apply for an M.S. in Statistics?
If I were in your position, I would look at the requirements for admission to Master's programs at universities you're interested in and see if you can find a way to fit their requirements into your required curriculum. 

For instance, here are the admission requirements at my university. I'm not sure if it's top-10, but definitely top-20. At this school a degree in math is not required for admission to grad school. It looks like they require up to an analysis course and some abstract algebra. This is likely only a few classes beyond your required calc+linear algebra classes for your economics degree.