Does anybody know how to make the coating on Sugar Crisp cereal and how to apply it?

Its just pure sugar, maybe a little honey. Its not healthy, put it that way.

You don't have to replicate it exactly, to make a nice cereal.

Why not get some unsweetened puffed wheat, drizzle over a little honey, and some sliced banana. Sounds much nicer to me, and its a lot better for your teeth, and still cheaper!

Make a caramel.

I'm pretty sure it's 3 parts sugar to 1 part water, and just let it simmer until it turns a nice golden color, and use it to coat the cereal. I'd use brown or raw sugar for more flavor.

This is what I'd do, make the caramel.

I'd cover my kitchen table or counter top space with wax paper, and spread the puffed wheat on it and use a fork or a whisk to drizzle the caramel over the layer of cereal. Once the caramel dries and solidifies I'd just vacuum seal the cereal.

If you want it more like the cereals you mentioned, you'll need a thinner caramel so make sure its thin, you want a light coating. So just have your puffed wheat in a bowl and while you stir it around slowly pour in your thin caramel until it's coated and lay on a baking sheet to set. The problem with this method is the cereal will most likely stick together and separating it all without grinding it into powder might be a bit challenging.

You could use the first method for lots of different cereals and even do it with white or milk chocolate. (obviously not crazy amounts for breakfast cereal)

You can also flavor your caramel with some vanilla bean or cinnamon and a whole variety of extracts.

You can buy freeze dried fruit and granola in bulk and make your own custom designed cereals.

Way cheaper and you get exactly what you want.

I'm guessing it would be done the same way you make candied or glazed walnuts. Use a simple syrup solution and boil it until it reaches the softball stage. Toss in the cereal to coat and then spread it on a sheet pan to let the sugar harden.

Put a couple of cups of sugar in a pan with about a cup of water and bring it to a boil. Without stirring, bring the sugar/water solution (simple syrup) to 235 F. This is the temperature at which the softball stage is reached. You can use a candy thermometer to check the temperature. Add about a tsp of vanilla extract when it reaches this temp. Pull it off the heat and quickly stir in the cereal and spread it on a sheet pan.

Problem with creating a caramel is just that: it's a caramel. It's nowhere near the same as the "frosted" nature of some cereals. Doughnut/dessert glazes (powdered sugar+water) seems closer to what you see on each flake. You may very well be able to dust the cereal with powdered sugar to achieve a similar effect.