Sadly in overly price sensitive markets like India, reliable brands like OS and Futaba doesn't have proper support.
The Piston and Case assembly is overly prices to be replaced. Try on e-bay or best, Tower Hobbies. Shipping form Towers would be not much ($9 for this) and their parcels usually reach without custom hassle.
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXNXD5&P=7But I would still suggest you to eye for a used OS 55 Engine on RCG/RCU/Ebay instead of paying $100 for the Piston assembly.