A previous thread mentions checking Hitchcock's and Burton's Bike Bits in the UK as well as checking with our host and a number of other sources here in the USA. You can get pistons, rings, gaskets, and some ignition parts here in the US if you know the right people to check with. Hard parts like engine cases, crankshafts, & transmission cases can also be found if you have patience.
If you post the engine number on the left side of the engine case just below the cylinder base, I can narrow your year model down to within several months of manufacture. The number may have three or four letters like SMPA for example followed by a four digit number. The rear early ones may have the number on the front of the engine case near the motor plates and it would begin with letter "T" or "W" or something like that. The Royal Enfield Owner's Club in the UK has a dating service on aproximately when the frame was made and who the original purchaser was. The frame number is on the headstock and is usually just a four digit number. Most Indians were purchased by Brockhouse for transfer to Indian Sales Corp as an exporter.
Good Luck and feel free to post any questions you might have to get you started.