In addition, how can I increase the speed of the motor
That will be in the code, answer: probably. You have to look in the code where the speed is controlled and see.
What is the use of the crystal between the pins XTAL1 and XTAL2 of 8052?
Read the '51 "bible" address in links <---
If I replaced it into higher frequency, then can I increase the speed of the motor?
Probably; however, you can only go as high as the datasheet for your particular derivative allow.

Erik