What needs to be done is to stop the progression of rust. If you go to a denter or do any tapering with that area, it will constitute chassis number alteration (which as you know is illegal) even though you are not doing so.
I would suggest to apply a chemical over the rust that will convert rust (iron oxide) to iron phosphate. This a very old tried and trusted solution to stop rust in its progression. I personally have used this chemical and vouch for its effectiveness. The brand I used is not available in pakistan but they basically all work the same.
https://kktraderspakistan.com/product/rust-converter/
You should do a bit of research on rust converter to gain more knowledge.