I think any magic that deals with osCommerce can be considered Deep Magic, and not fit for mere mortals.
1050 attributes.... How long does it take to load the attribute edit page for products? uc_attribute.module was designed with that page in mind, as an anti-pattern.
What I can do is show you how osCommerce tables map to Ãœbercart tables. That should help you figure out which values go in which tags in the XML file.
| osCommerce | Ubercart |
| products_options | uc_attributes |
| products_options_values | uc_attribute_options |
| products_options_values_to_products_options | uc_attribute_options |
| products_attributes | uc_product_attributes |
| products_attributes | uc_product_options |
It's probably a good idea to make a few test attributes and products to see where all the data goes. osCommerce doesn't have any equivalent to the price and weight fields in uc_attribute_options, but if most of your products use the same values for those options, you can figure out what to put there.
Good luck.