Hello,
This is a common problem with all old M800 owners. Your floor is still in a better condition. When I had bought my car and removed the carpet I could easily see the road below and there were big holes. I have done lots of tinkering and welding work.
See my ownership review.
http://www.theautomotiveindia.com/forums/ownership-reviews/7693-my-first-car-maruti-800-ac-93-model.html
For your car's floor I have a cheap and reliable solution. You can do it as a DIY.
First of all check the floor for any huge cracks or holes. If there are any huge cracks/holes then you'll have to get welding work done, else you can use this method.
Material required.
1. 100 no. sand paper Rs. 20 (for making the surface smooth)
2. small wire brush Rs. 20 (for scrapping the old rust)
3. small tin sheets (used by painters) Rs 10 (for applying body filler)
4. body filler Rs. 200
5. Red oxide primer Rs. 70
6. Paint Rs. 30
7. Brush Rs. 10
total Rs 360
First of all you will have to scrap all the rust using a wirebrush.
Then sand all the uneven surfaces caused due to rusting.
using the tin sheets fill all the cracks and holes with body filler.
let the body filler dry for one day, then apply a thin coat of primer all over the floor. Let the thin coat dry and apply a thick coat of primer. Let the primer dry and then apply two coats of paint over it. You can do this on the floor and underbody.