Can a 120mm fork be mounted on Specialized Myka?
Most 120mm forks have a similar axle to crown height and it's more or less 40mm higher than the axle to crown height of typical 80mm fork. There are of course differences between models, but not really enough to matter. 

Typically hard tail frames are designed around a range of fork sizes. 120mm would be long for an XC frame that came with an 80mm fork. I doubt the manufacturer would recommend this, however lot's of people do those kinds of swaps without significant issues. It is pushing the design limits of the frame, and if you're pushing the limits of what the bike can do, you may run into problems.

That big a change will affect the geometry and handling of the bike significantly. It will slacken the head angle, raise the front handlebars and also rotate the seat tube backwards. These are all great if you want to go downhill fast, they are all bad if you want to climb uphill fast. It will also lengthen the wheelbase and make the bike a bit more difficult to handle in tight corners at slow speeds. 

Setting the longer fork at a lower sag value ( i.e. 25% vs the more typical 20% recommended for XC use ) will help limit some of the geometry changes.