How to replace marshmallows in rice krispies cakes?
There are plenty of recipes for rice krispie cakes without marshmallows in them. Rather than trying to substitute in your recipe, you'd be better off making something slightly different but that's been tested. 

Here's an example from tesco. The recipe we use is based on golden syrup, but isn't online. A search for "rice Krispie cake -marshmallow", possibly with the addition of "-chocolate" will find you a decent selection.