Thanks for the vote of confidence 
1. Fuel consumption on CNG with 1300 cc engine
No idea sorry since I dont run any of my cars on CNG. My 1200cc daily driver bug gives me easily 10km to the liter in city traffic, the 1300 should come close, maybe 8-9km/L?? Engine must be in good shape and more importantly tuned well to be fuel efficient.
2. Cost to overhaul the engine
A full overhaul used to cost around 25-30 thousand rupees. Of course more if you need to replace more things than usually are required. For example if the Crankshaft is shot, cost rises. So on.
3. What major mechanical work is required to run in in smooth condition with cost
Depends on the particular car you get. If you ask me, its best you get all mechanical work done under your own supervision provided you know what to do and more importantly what not to do.
4. where to buy a running beetle car
Your luck. Look around, newspapers adverts, ask mechanics, ask friends, check adverts on this website.
5. will it create heating problem in summers
Biggest myth!!! Absolutely nauseates me :o. The VW is designed to run efficiently in a desert as well as under subzero temperatures. Our pea brained mechanics and sadly very many people just cannot digest the fact that its possible to keep something cool by using something other than water. A stock engine which has not been messed around with by a Mr-know-it-all-ustaad-jee should keep running good under all weather conditions.
5. what is the monthly maintainance cost
In a well rebuilt car, fuel, oil for oil changes, air for the tyres, keeping the body clean and.......well thats about it.......
Hope that helps,
Best regards.
