Don is giving me too much credit, although I have done a lot of work on my Motobecane model 7's, I have not needed to open up the engine at all.
I did not even recognize this model as a Motobecane!
That said, I checked my manual and it is a little ambiguous on piston removal, it talks about using a piston pin removal tool and heat to re-install it unless it is a later model which is an easy sliding fit. It also mentions that if you heat the piston, you should slide the cylinder over the piston, without rings, to cool it off without causing distortion. The rings may or may not cover the locating pins so watch that.
Obviosly you have to remove the circlips from both sides of the piston pin befor removal and replace them after. Make sure the pin is centered so the clips seat on the groove and do not apply side loads to the piston/rod assembly as you may bend the rod. The piston pin does use a caged roller bearing but no mention is made of the rollers falling out of the cage. Be sure to lube everything with 2 cycle oil during assembly.
Hope this helps.