The window in the piston is the most important part, if there was no window in the piston (or cut-away from the piston skirt) then you are not increasing your intake timing duration, witch is -among other things- what converting to a reed valve would allow you too do.
Alternatively, if a "boost port" is cut from the bottom of the cylinder bore (underneath the intake port) up through the intake port, and out the top, to the same height as the transfer ports, the piston modification is not required, but will still provide an improvement in overall flow, and performance.
The "boost port" also known as -in our case- the third transfer port (cut from the intake port, up, to the height of the transfer ports) increases the performance gained by creating another path for this extra intake charge (if you do indeed increase the intake timing) to get into the cylinder.
Reed Valve conversion without Piston Window or Boost Port = Meh
Reed Valve conversion with Piston Window and no Boost Port = Improved low end power, possible loss in top end (depending on window size/shape)
Reed valve conversion with Piston Window and Boost Port = Even more improved low end power, with more top end power too.
If you have any other question don't hesitate to PM me.