It all depends on how bad the scratches are. If the tank is dry, you would be better off using some power tools to polish it. IME, the wet, hand powered kits end up making things worse.
To make teeth a table saw or router would be easiest.
For cutting teeth in an overflow, a router with a dovetail guide is the ticket. A table saw would work, but it would look like something out of a horror film!! ; )
I've also seen a DIY'er cut an overflow box WITHOUT teeth so that water just ran over the top. He did, however, cut a piece of eggcrate to form a grate for the water to pass thru and stop larger debris from getting thru. His method actually looked pretty good.
The problem with using a table saw is that the saw blade is really thin, so you'll have to cut tons of slots to get decent flow. Then there's the problem of consistancy: slot length, and distance b/w slots.
OK, so I'm a perfectionist!!!!!
You can see the outcome of the overflows that I make using the router method on my website, if you're interested.
Either way, good luck!