Yeah you can switch the sleeve and piston from a 2.5r to a 2.5. There isnt much of a differene beside the apperance of the heat sink. Infact they dont even make 2.5's any more from what I have heard. The only difference from the 2.5 to the 2.5r is the port's shape and the port timing of the engine. Thats the only real power difference between the two, you can easily switch between the two and they should fit.

As said they will fit the only real difference between the two is the apperance of the heat sink and some little port mods that traxxas made. But if you want to buy a new engine then look at O.S, there better then the Traxxas engine's in many ways. There more reliable, easier to tune and hold there tune for forever, and a .18cvr makes way more power then the 2.5 does. But you will have to do some mods to make it fit on the truck that you have,but I think the mods would be well worth it and it only cost's $99 on towers the last time I checked. Or you can get the .18TM also made by O.S and that is able to use the ez-start. The TM is as powerfull as the 3.3 is.