There's a bus from MGM to the TTC. You can take that and then either take the ferry or the monorail to the MK. I'd allow an hour from the MGM bus stop.
An alternative would be to take the bus to the Contemporary Resort and then walk to the MK (walk is 10-15 minutes to MK entrance), or take the bus to the GF or Poly and jump on the resort monorail to the MK.
If it were me, I'd get on whatever bus (of the ones listed above) shows up at MGM first.
No matter which route you choose, allow about an hour. It may take less than that if you don't have to wait long for a bus or monorail.
Best wishes -