There is NOTHING that will make the RT100 fly like the KX100. The RT is basically the same engine as the older DT100's and is air cooled with lower compression, different transmission and heavier. I LOVED my DT100 when I was younger and it was great but it wouldn't even keep up with a YZ80 of the same year despite having 20cc's more.
If you want to fly like a KX then buy a KX. The RT was made to be an easier to ride entry level bike and it fits that need very well. It was not made to jump doubles and doing so will result in broken bones and possible loss of tonails (speaking from experience).