For wot my 6 eggs is worth....
From my small CS experience there's no problem with fitting Indian spares to to a Dursley lump, India was part of the British Empire and they are quite happy with feet and inches and Imperial threads.
I have a bottom end that left the factory as a 3/1 hopper cooled in '34 (I think) that came to me with the larger barrel (Jug to you. cousin) iron piston, head ect.
Once I had it running, with Indian injector and pump, rings, white metal mains, +0.060" big end on a heavily re ground crank and mostly Asian linkage I swapped the iron piston for an Indian alloy one in a (very successful) attempt to overcome the balance issues, caused I believe by the flywheels being weighted for the smaller, lighter 3/1 piston.
I then changed to Indian taper roller bearings as the crank was well worn, fitting the TRB's with Loctite 620 to take up the slack.
I've only put half a dozen hours on it so far, driving a Stamford 8 KVA head at 600 rpm but it's looking good so far.
Basically it's a Dursley / Indian bitsa, every thing looks good and works as it should, so..........
If there's guys in the UK willing to send you stuff at the right price would it not be worth while having a crank case filled with small parts, piston, con rod, injector and pump freighted to you as spares and another, crank, flywheels, barrel and head, rest of the small parts sent to your brother, uncle or best mate in the next county a few weeks later also labeled spares?
You'd then have genuine Dursley quality on the castings, that would have no more 'Indian' content than most of the one's still running today which, I would presume, would have more intrinsic value than a rough clone, without importing a complete engine.
Not that I condone getting around irritating legality in any way shape or form, much!
Cheers Stef
