Turntable plays 33.5 Lps very slow

I agree with rick. A belt will stretch out with age. I am curious what kind of powder did you use? Odd the 45rpm platter speed was fine before you used the powder. By chance did you use this powder when you first set up the TT?

Kal, the rest of us in this thread new he ment 33-1/3.
Akhan, if the powder you used was baby powder it may have moisturizing oils in it. Make sure you use 100% pure talcum powder. If you did use a baby powder with a moisturizing you will need to clean the belt to get off the oil. To clean this old belt you could use a clean tissue soaked in methylalcohol, {wood alcohol}, Not rubbing alcohol. Also reclean the motor pulley and platter edge. This might get you buy until you get a new belt.
Good luck with your new toy.