Did you say Ontario? So that means you cannot (legally speaking) use herbicides. In that case, your original thought--sod--is the best course of action. Wait till about August 15 as the weather is more suitable and most weeds and crabgrass are no longer a problem. If you are in the frozen north--wait until the fall daily high temps fall to below 80, (26.6 C). If that is late July--fine. Remove the present weeds and crabgrass any way you can--flame thrower--dry ice--vinegar--black plastic--whatever works for you. Naturally, correct the problems that caused the weeds in the first place. And...keep the new sod well watered--during establishment--and thereafter also. Sod grass--(if high quality)--will be denser and resist weed invasion.