Yes exhaust. Extractor design does not matter much when it comes to runner length. Combustion events are 120deg appart so no scavenging. If the shape is smooth and looks good it will probably work well. My exhaust is 1 7/8", however, it goes straight from front to rear without having to go around the fuel tank. 1 3/4" or 2" would be great (bigger may only benefit higher rpm). More compression would be great. I am at 68fwhp with std pistons.
Adjustable fuel pressure reg and wide band O2 with an AEM or Innovate style meter will help to tune it a little. AFR should end up about 13:1 at WOT.
I added a alu adjustable cam sprocket. Retarding the cam 5deg or so is something it liked. The gear was cheap through Aliexpress and seems extremely well made.
If doing the pistons, I’d at least get the combustion chambers tided up of all sharp edges. There will be no downside to the tiny loss of compression, instead you limit knock and get a more powerful/quick combustion event. If you happen to have the valves out, tidy just inside to blend the valve seats to the ports, as a minimum.
I machined my flywheel down to 3.5kg. Big difference in how the thing felt with that. As power is a function of torque multiplied by engine speed less reciprocating weight means more power as it can spin up faster.
I’ve not done pistons yet. But the plan is to use some GSXR600 pistons with custom rods. The bits are all sitting there for “when I get around to it”