I've worked on may 125 GP bike engines in my time and a simular piece in the middle of the port is for the ring ends to be supported as they pass the port. Are the ring ends in line with this piece? If not, I don't know what it is for either, get rid! The other thing to watch is if and when you do this, because the port is then quite alot more open, the ring will ,as previosly mentioned, need quite good lead back onto the cylinder wall. In addition to this (stay with me) you will need to put a small chamfer onto the top edge of the piston ring groove. This is because the piston will try and burr over the piston ring and result in the ring sticking into the piston. By putting the small chamfer on it, the burr dosen't occur as you have already removed the material where it would occur. You will never suffer ring stick if this is done properly.:hmm3grin2orange:
I know this is hard to follow, so if you need proper details, then get back to me and I will post some pic's and instruction I have from my racing days.
Andy