Some comments:
The Athena exhaust has only a 19mm stinger, I replaced that with 22mm (ID) stinger, and it improved a bit in the mid to high revs. Also helps keep temperatures down. The silences also has a 'restriction' in the perforated pipe, so it would be good to replace that too (even if I didn't)
I found raising the cylinder also helps a bit. I found 1,5mm to be a bit too much, 0,5-1mm seems to work. You also need to adjust the head when raising it. I cut my own 'bath tub' chamber with abt 11:1 geometric CR, which seems to be a good compromise to avoid deto with pump gas. At the same time I changed the shape of the piston crown to a 'normal' shape, not the Athena compromise so they could use the same head as the 125 kit.. even the power valve seems to be the same, radius is not matching the 65mm bore..
I definitely recommend Zeel, it will give you the possibility to tune ignition to get some more over-rev, tune ypvs to the pipe etc.
Seems to be the transfers that limit the high rev power on the cylinder. Raising the cylinder improves this, because the transfers are far from fully open at BDC. However raising the cylinder also raises the exhaust to extreme durations (approaching 200deg when you start raising the cylinder), and that is not good. As is, the exhaust blowdown STA seems to have margin for more power, but the transfers don't flow enough to support it. So if you are going for porting, I would recommend starting with widening and raising the transfer port windows. I don't think there is much to gain from matching in the crank case etc, it might make you feel better but rarely has any real results unless it's really bad.
