I think you'd probably need to get rid of Mulroney for this to work.
If by "1988" you mean the '88 election, maybe have Mulroney announce his retirement shortly afterwards, saying something like "Well, free trade is my legacy, and now that that's secured, I can make my graceful exit."
Or maybe the Tories somehow lose that election, but with a respectable showing, and thus live to fight another day.
Also, having the CF-18 decision go the other way MIGHT help thwart the development of the Reform Party, who were sidelined by free-trade in '88 but of course came back with a vengeance in '93. But there were other "western alienation" issues at the time, so who knows.