the bike is HASA F-SL which has only one chainring at 47T and one single cog at 9T at freewheel. I have seen pictures of someone adding a 39T to the big ring to bring down the gear ratio (see picture) the original 47T becomes unused (since it is attached to the crank) and just shield the smaller 39T chainring. I know that the chainring is a BCD130 5-holes. If I want to do something similar, is it a simple operation (I have no experience in bike modifications before) by removing the crankset and bolting the small ring to the back of the big ring? Or does it involve replacing/adding a whole bunch of other components? I have attached a picture of the spec if that helps.
If such operation is relatively simple, instead of a typical 39T circular ring, can I use a 39T Rotor Oval ring (and pair it with the soon-to-be decorative big circular ring)? Please note that there won't be any derailleur involved.
Or, can i even get rid of the big ring (but keep the crank) and just have ther oval ring replacing the big circular ring?
