How can I access the Instance.new() base script in roblox?

With OOP, your best bet is probably just organizing your Sword class, including the constructor, in a (or some) ModuleScripts.

As far as I know, I don't think there is a way to edit the Instance.new() function.

I don't believe Roblox allows access to the source code for their API, although I could be wrong. Anyhow, you could create a sort of wrapper around the Instance class using metatables/metamethods. They are pretty much the closest thing Lua has to OOP, and it's likely Roblox used them to make the objects in their API. Here's a link in case you don't know about them:

http://wiki.roblox.com/index.php?title=User:Thenewguy/How_does_ROBLOX_use_a_metatable

I mean, you could just make the sword a model then use LoadAsset instead of Instance.new.

You can overwrite the global Instance variable with your own table that indexes the real Instance class if necessary. You can then write your own 'new' function into the table.

local Instance = setmetatable({}, {__index = Instance})

local realNew = Instance.new
Instance.new = function(name, parent)
    if script:FindFirstChild(name) then
        local instance = script[name]:Clone()   --  Don't store stuff in the script, configure accordingly.
        instance.Parent = parent
        return instance
    end
    return realNew(name)    --  exclude the parent argument as it is deprecated
end


Instance.new("Sword", workspace)

If you wrap that into a module you can simply do

local Instance = require(extendedInstanceClass)

at the beginning of all of your scripts.

This is a very simple way to extend other classes and libraries as well. Although, you do need to do a little extra playing with metamethods if you want to be able to call or write members of those extended classes. With the Instance class it doesn't matter though, as you will never be able to directly index and call Instance.FindFirstChild for example.