The YZ85 is a modern race bike that has a much more radical tune and advanced design compared to your RT100. The little RT is very similar to the mid 1970's DT100, and bikes have changed alot since then. The YZ will make much more power even though it has less displacement, and it's suspension and frame are also far more advanced.
You have a fine beginner's bike - very similar to what many of us learned on. Enjoy it, and learn the basics about riding, maintenance, and bike set up for the next few years.
You can stiffen your forks with some minor changes (adding some fork oil will improve the bottoming resistance), but I would resist the urge to make costly modifications to the RT.