Simply put, the following are a typical drivetrain that allows easy adjustment of speed
Propeller
Shaft/stuffing tube
Motor & either dogbones for direct drive or gearbox for a gear reduction motor to propeller
Electronic Speed Control (throttle and can limit top end speed)
Sizing those is best done by finding what someone else has that works and copying the prop/motor/gear box or esc, as optimum propeller choice for us is a very experimental process