With a non-vibrating crank and a good CDI with a true 2 stroke timing curve you can rev up to 8500 RPM with an exhaust port open duration of 175 and a transfer port duration of 117. If your intake is still piston port (w/o reed valve) then you also need a 120 degree intake port duration. This shows you the distances needed from ports top edge to the point on the cylinder where the top piston edge is at the top of its stroke:

So if your piston has a stroke of 39.5mm then you need 22.8mm from exhaust port to TDC to have a port duration of 175. Just take the head off and rotate the engine till piston is at its top position and then mark the cylinder with a marks-a-lot. Then take the cylinder off and use your rotary tool and cutting discs to grind the tops of the ports till they are the right distance from TDC. You can use your digital caliper to measure the piston stroke as you rotate the engine with the head off. 175/117 durations is shown by my porting calculator to be just right to have a broad powerband for use on the street. With the limited transfer port area its just not practical to try to get it to rev higher than 8500. If you have an expansion chamber then the centerline distance from piston face (at exhaust port) to the beginning of the baffle (the contracting cone) should be 33 inches (840mm). Shorter than that is for a higher top RPM but you probably won't acheive it since the engines peak power RPM is only about 6800 with this porting. The pipes top RPM can't be too much higher than the portings peak power RPM or that non-synchronous arrangement will cause it to fall off in power before it gets into the pipe powerband (usually the last 1500-2000 RPM).

Here's some of my YouTube videos that may help you with porting:
Methods to find port durations
Porting Rules
More About Porting
About 2 Stroke Ignition Timing (by CDI)

Here's my page about measuring to find durations: Click here

The whole idea I maintain when modifying is that a single speed bike needs an engine that will provide the broadest powerband possible. At 8500 RPM with 26" tires and a stock 36 tooth rear sprocket your top speed will be around 43mph. For higher speeds use a 32 tooth sprocket, and for lower speeds (but better ability to climb hills) use a 40 tooth sprocket. You can find these on Amazon.

Click here to read about the significance of port shapes.