I'm not aware of any love story between Sonakshi Sinha and Zaheer Iqbal. It could be that some rumors might have spread, but without any substantial proof like them being seen in a romantic context frequently or any statements from either of them about a relationship, it's safe to say there is no known love story.
The 'lemon' aspect might be a bit hard to come by in a clean way as it often involves inappropriate content. But for normal Korra and Mako fanfictions, you could also try FanFiction.net. Some stories there might show their relationship developing, their adventures together, and how they support each other, which can be really interesting to read.
Korra could meet the founders of the different bending disciplines. She could meet the first water benders and learn about the purest form of water bending. This would not only enhance her own skills but also give her a deeper connection to her water - bending heritage.
Archive of Our Own is a great place to start. It has a large collection of fanfictions, and you can easily filter out any inappropriate content. Just search for Korra and Mako, and you'll get a variety of stories exploring their relationship, from friendship to romance, all within the boundaries of a proper and respectful narrative.
There are no solid facts regarding a love story between them. It might be just baseless rumors that sometimes float around in the entertainment industry.