I think "piston rod" is referring to the connecting rod. Either that or the piston pin. The only parts you need to do the complete upgrade are the parts listed here. This is pretty much all the working parts inside the motor except the crank and a couple bearings. Here are the part numbers again:

(G70344-08) CEN Piston and Cylinder
(G70344-07) CEN Connecting Rod
(G70344-04) CEN Piston Pin

The easiest place to get all the part numbers is Toyhaus. Here is a link to the Funfactor MT2 ordering diagram:

http://www.toyhaus.com/ff4wdmt.html

Toyhaus has great service and stocks most every CEN part available. But, expect to pay full retail or close to it. I was a little nervous when I did my upgrade, but there really are not that many parts to these motors. Just pay attention to how you take it apart and you shouldn't have much of a problem. If you get stuck, just ask. There are some really good people here that have helped me a bunch.