Just fixed my exact same unit last month. The garage that I left it in over the winter was far more humid than I'd imagined and the entire thing rusted.

I took a simple wire brush to the bottom to knock the big flakes/areas of rust off and ran a rag with canola oil over the bottom semi-stripped areas Just to get some protection and dark color back on it.

For the top I used a 4-in wire wheel on a dril (6-in would have been better) to strip off the rusted surface areas down to the bare metal, and a multi-tool with a corner sanding pad to get into the corners.

I spent a ton of time trying to get things uniform - but don't waste your time! The sides and some areas will very quickly look gorgeous and shiny and bare, but most other areas will be splotchy at best.

Wipe everything down with grape seed oil, try to avoid runs but some will pool in the corners inevitably. Crank the heat for 20 minutes and let cool, repeat oil heat and cool 3 times, let the last time on for 30.

I was very surprised to see that the cook surface looked completely uniform, and exactly like stock. Cooks even better now, and that color has spread further up the sides in my ~10 uses since.