Even though you do not have the original files, you still have an admin, and that is where you will control the majority of your SEO. You will also need the admin to Add, Edit, and Delete, Categories, Products, etc...
Sounds like you might not have the login info for the admin? If not, you could try the default login info, in the off chance that they did not change or remove it. After the .com of the site, type /Administration. For the email address, the default address is '
[email protected]', and the default pasword is 'admin'. If you don't have the login info and the default login info does not work, I am not sure how you would be able to get in. Maybe someone from the NopCommerce team could help with that.
If you had say 20 products, the Products page is actually just one page, that gets populated from data in the database based on the product you choose.
So the only pages you could do the SEO by hand with, would be pages that are static, and those should still be using what is called a MasterPage. This is actually ASP.NET, not classic asp, and it is a whole new ballgame than html and classic asp.
I hope that all makes sense, and let me know if I have helped in anyway, and if I can help any further.