As long as the old phone takes SIM cards, you should be fine :) You can purchased a prepaid SIM here: https://shop.koodomobile.com/phones/a... (or in a Koodo store, obviously).
Make sure you get the $20 SIM card as the other one won't work. You'll get that back in a $20 voucher though :)
Edit: oh I don't think the Keybo phone takes SIM cards... it mentions "CMDA" which would not be compatible :( Sorry! You'd need a new phone with your prepaid I'm afraid.