Stock is best all around power for 50cc.
Since you are 70cc, you have the torque to get you up onto a high rpm pipe.
Looking at the geometry from a pipe builders standpoint:
Proma is out of stock, but looks like it has a wider power band than the 50cc homoet, but peaks around the same rpm.
The tecno also looks like a high rpm pipe, and the bend shouldn't hurt performance too much.
If it were me, I would opt for the 70cc homoet or the proma with a 70cc mb5.