The reserve powers of the Crown have been reduced even more in Canada than in the UK, and there are several cases in just the last decade where the Lieutenant Governor or Governor General has had to make a decision. For example, if there's a motion of no confidence in the first sitting of the legislature, and the Prime Minister loses power immediately, the right answer is contextual. Sometimes another election really would be appropriate, in many cases the opposition can form a working government, if given a chance.

None of these problems are insurmountable, of course. And you could just keep an executive -- I personally prefer just replacing the Governor General in Canada with a president, preferably indirectly elected. Still, patching the constitution would be pretty involved, if you wish to fully excise the executive in its current form from the Westminster system.