The sort by SKU could be a useful option. You can suggest and vote for the idea at
http://ideas.x-cart.com/ - when choosing features for new X-Cart 5 versions the dev team considers votes given there for improvement ideas.
As for the sort order number inside the product. How should it work? It looks like the "Default" sort option is the one that you are looking for - it allows you to configure the order in which products should appear in particular categories.