Substitute amaretto syrup for amaretto liqueur

Regarding the amount of "heat" in the recipe, I'd agree with @justkt that you'd want to go with 1/8 to 1/4 (at most) of the specified amount of black pepper if substituting cayenne. I like spicy food and go through a lot of cayenne (and other) chile pepper preparations, and they can vary a lot in terms of strength even within the same variety. So, I'd start on the light end and add more to taste if it's not spicy enough for you.

My wholly unscientific opinion is that when a recipe calls for black pepper in any significant quantity it's for the flavor as much or more than the spicy heat. If you have any ground cardamom on hand you might could toss a pinch of that in as well to replace some of the resinous flavor that black pepper has and cayenne lacks.

Props to Magnus for his excellent and botanically accurate answer.

When heating sugar up in boiled icing or in making candy, the problem is sugar crystallization. This happens because the solution becomes supersaturated and any movement can cause it to shift back into a crystal state.

The corn syrup is there to prevent this from happening by providing glucose to 'get in the way'. You can get just 'glucose' at the drugstore to replace the corn syrup. Any other sugary thing that's high in fructose or glucose should work.

Another option is an acid. Some acid (like lemon juice) should make some of the sucrose break apart into simpler sugars and prevent crystallization.