Oil tanks from India

There's no doubt an interesting back-story as to how the grit got in the tank in the first place, but have you tried a power washer?  You would also need to to reverse flush the internal metal oil return pipe out (just inside the top of the tank) very carefully, but any doubts at all then please flatten it with a hammer and take it out of, erm, circulation.  I consider the blasting of any engine etc parts, including vapour blasting, the work of the Devil.

Did you have an oil filter fitted?  Pretty essential really, though of course it wont prevent any existing grit in the tank getting into the engine, as it has to be fitted to the oil return pipe from the engine unless you are very clever and skilled.

I have no experience of an Indian one, but as I assume that they are hand made on a one-off basis the quality must vary, and returning one to India might be a hassle.  How risk-adverse are you?

I do know that the original factory tank has earned a reputation for the mounting brackets failing, and the bottom mounting bolt creating local cracks in the base.  (Regular best quality rubber bobbin and bottom bolt grommet replacement helps).

Whether AN has upgraded the original design and material specifications I don't know, (and they are needed), but whatever you replace yours with needs to be top class if you want, erm, reliability.  For me that would mean AN, as they are still available.

What are you using to flush it out? Warm paraffin or cold (you don't want to heat it up) petrol are pretty effective. For the first few rinses you can filter the mucky solvent through a cloth before fresh solvent every time as you approach gleaming perfection. A gouple of gallons of petrol or paraffin have to be cheaper than a new oil tank.

...where did the grit come from? Was it from some process the bike /tank has undergone, or is it detritus from wear and tear inside the engine?
So what exactly is the root cause, and can you be certain it will not reoccur? 

I had to clean an original 750 fastback oil tank that had been blast cleaned at somepoint in the past. It had not been cleaned out of oil when it was blasted and all the holes had a build up of blast media stuck to the oil residue in and around the filler neck, oil filter outlet etc. even the outlet strainer had got full of media stuck to the oily parts.
Degreasing internally, pressure washering and into the ultrasonic tank they went. It is amazing where the media got to and how stuck it was, age might have something to do with this as to how long it had been like this is unknown. Inspection with a endoscope and repeating the treatment ensured a clean tank. The media got into the strainer because it had been in the blast cabinet by itself and the media was blown into it through the pipe outlet.
Just 'flushing the tank' won't clean it properly. 
Back to your engine, What engine is it and what is its history? did it have the tank strainer fitted? For grit to find its way into the engine it has to get past the strainer, then after the oil is pumped into the crank it has to get to the journals but in doing so any 'grit' would be thrown into the sludge trap in the middle of the crank itself. 
The oiltank after a lot of miles should be cleaned out/flushed and the mountings inspected for cracks, say every 10k.
Endoscopes are quite cheap these days, You can get ones that plug into your phone. Ultrasonic tanks, not so cheap.
            

Adam, another possible tack if all else fails, is to cut either the back or the bottom of the tank out and weld it back-on after a detailed cleaning.

Personally I'd probably cut the  bottom off, which would also allow you to modify and strengthen the fragile factory design should the new AN replacements prove not to have already been upgraded.  Then there's the brackets of course.

An endoscope sounds like a great idea, though it does bring to mind foreboding images of sitting in a totally silent Waiting Room at a Prostate or Colon Clinic