Do you have a chronoscope that you want to use or do you want to make one? With 2 LEDs and 2 phototransistors, you can measure the time between two events (each PT seeing a beam break). Knowing the distance between the two PTs will allow you to compute the average velocity between those two points.
To display the data on an LCD, you will need to interface it to a microcontroller which is responsible for formatting up the speed information and sending it to the LCD. There are hundreds of web sites that tell you how to do that. Look for 44780 (the ubuquitous character mode LCD controller) on google.
Voltage is pretty easy you need a microcontroller with an ADC. You will probably need to know the voltage range and then divide it down to something that the ADC can handle (<5V).