Would like to make my 100cc 2 stroke engine gain more speed

A gear change is the only way.
Seriously??? Then you truly don't understand propulsion.

At full throttle a certain HP/Torque/traction is going to propel a certain weight/aerodynamic design/vehicle engery loss a certain speed.

Without changing one or more of these factors speed isn't going to increase.

I have a shifter bike. On my engine, max hp is at 7000 rpm. Therefore if I'm doing 7000 rpm at full throttle on level ground shifting to a higher gear isn't going to make me go any faster. The increased workload is going to cause my engine's rpm to drop. Which is actually going to slow me down some because I'm below max hp rpm.

Outside of gear ratio (transmission), speed can be increased through weight reduction, aerodynamic design, tire selection, hub bearings, and engine performance.