Before we tell you 10 different engines that might work, tell us more about what you have.
Is it a side exhaust or rear exhaust? Is it a slide carb or rotary carb?
Answer those two and it will narrow the field quite a bit one way or the other.
The 2.5 maxx is a rear exhaust with a slide carb. The old 15 maxx is a side exhaust rotary carb.
In either case, OS has you covered with a couple great engines. For the side exhaust/rotary carb, the 18CV-R(X) would work really well. For the rear exhaust/slide carb, the 18TM would work easiest.
Both are great engines and are long life, easy to tune engines with much more power than their Traxxas equivalent.
With any engine, an upgraded pipe almost always helps with performance and reliability. But, if your trying to get going the easiest/cheapest, figure out what you have and let us know.