No Bake Cheesecake lemon juice instead of lemon extract?

It's a pretty perilous substitution- lemon juice is extremely acidic, while lemon oil (the flavor element in extract) is ph neutral. Lemon oil is available separately and is a considerably better buy than extracts usually are. It is also the flavor element in lemon zest, so I'd use that, unless you're sure that adding acidity won't throw the recipe off.

The lemon extract is probably both a small amount and not the only source of lemon flavor.
If that is the case, I would just eliminated it or replace with some other alcohol-based flavoring...vanilla or orange extract, fruit or nut-based liqueur, or brandy.
If the lemon extract was the only source of lemon flavor, yes use the juice and a bit of grated zest if you have it.