To what Steve P said about engine upgrade and all.......
but heres something he forgot to tell you. That chassis was not built with the more powerfully moderen engines in mind. So, if you upgrade in horsepower (a la new engine) you certainly will shorten the life of your already used, old transmission. and it also possible that you could tweek the chassis. Especially if you use that OS engine he suggested. That thing is has major torque, which could rip that old chassis to shreds. I'm not trying to scare you away from upgrading I just want to make sure you are aware of the pitfalls. that come with it.
(I've been down this route with and old MTX-2. It was fun but VERY costly. By the time I was finished I could have bought a new car)
Now you could contact HPI Harry and see if he has some hop-up parts that he could sell you or recommend a place that still sell them. Things like the rear and center braces that could help keep the car rigid in the transmission area. Of course, this adds to the cost of your upgrade and soon you'll see its just not worth it.
So unless one of the "old-school" HPI guys is willing to give you some free upgrades I suggest you just play with this car as is and then buy a more modern car when this on dies. Good luck with that and let us know how it goes.
My five cents......
Here is a link to a site that had some chassis upgrades for that car. You can use them for reference, since I'm not sure they're still in business. http://www.megatech.com/hpi_nitro_rs4.html