The TTR Pro .15 is basically a O.S. CV .15 copy, except it has a chrome sleeve (which is better). You should just get a backplate for any O.S. Cv for it ($4.95) . The reason it broke is that the rod DOES run against the backplate to keep it in place as the crank "walks" back and forth and with the first backplate it would run off the pin and break! You have to grind the pullstart pin off even then install a regular backplate. The backplate also serves to "stuff" the crankcase for better velocity of the air/fuel mixture.
The hole in the piston is a boost port that feeds the middle port in the sleeve since there is no case port feeding it, if you face it to the exhaust it will vent all pressure in the case and lean the engine or it won't run at all.
Tip: always mark the rod with a small permanent marker dot on the side facing you before removing from the engine so you know how to re-install it. Also never mess with an engine if you don't know what your doing unless you don't care if it is ruined. Always ask the help of a knowledgable person at the hobby store or at your local track.
Hope this helps!