Lye will clean up most grease, grime, etc. It won't do anything for rust. On something that large the easiest thing to do would be get a couple cans of EZ Off oven cleaner, you have to use the high strength cans with the yellow lids, they are the ones that contain lye. Spray it down real good and put a trash bag around it to help keep it wet. You might have to respray it every couple days but after a week or so most of the stuff should come off.
If it was me once I had it cleaned I would spray it down with Pam cooking spray and then build a fire and heat it up for an hour or so. When you do that I would spray it down then wipe it down good before heating it up. If you have too much Pam on it it will become sticky, spray it down and wipe it with a towel or T shirt until it looks dry, then heat it up.
I have been looking for a large kettle myself, post some pics of the process if you can.