The story starts in India where in 1992 Sooraj Automobiles of Saharonpur in Uttar Pradesh started installing 325cc Greaves Lombardini engines in the frame of Royal Enfield bullets. Greaves Lombardini was an Indian affiliate of Lombardini of Italy. This motorcycle, the Taurus, was marketed through the official Indian Enfield (later Royal Enfield Motors) dealer network in India until the year 2001. There were some tweaks done to the model during the years of production, including a version more oriented towards farmers, with a funny crashguard and in yellow, I have seen claims that a larger 400+cc engine was also fitted in these "production" bikes, but I haven't been able to confirm it (it might be confusion with the Robin Enfield, see below). The 325cc put out 6.5hp@3600rpm and a claimed 190 miles per (US)gallon at 25mph. Although diesel engines usually have a lot of torque, the Taurus' was a modest 15 Nm (11.1 ft. lbs) @ 2500 rpm (by comparison the Bullet 350 has 32Nm and 18hp). Apparently Sooraj also marketed some of these machines under its own brand name, and in some cases replaced the gas tank with a different model than the Enfield one.
Some reports exist that "private conversions" of Bullets to run on diesel engines like the Lombardini, which were in widespread use in India for irrigation, etc, had been done for a while and that this gave Sooraj motors the idea of "why not have production of these types of bikes" (see e.g. Wikipedia). It sounds entirely plausible that such conversions existed, given the large production of Bullets over the years, and the general ingenuity in India. An additional enticing element was that, unlike in the US, diesel is (and especially was) considerably cheaper (and better quality) than gasoline in India. According again to Wikipedia, the Taurus production was ended because of new pollution laws in India.
At almost the same time in the UK, Ernie Dorsett, who had done private conversions of Matchless motorcycles to diesel, got in touch with Redbreast (Blixworth, Northamptonshire), the UK importer of the Japanese Fuji Robin diesel engine. A deal was reached with Redbreast to provide engines and Banavar Products (at that time the importer of Royal Enfields) to provide rolling chassis. The Enfield Robin D-R 400D was powered by an all-alloy air-cooled Fuji Robin single cylinder engine of 412cc of displacement. It put out 8.5hp@3600rpm. It had a kickstarter but also an electric starter, allegedly being the first "production Enfield" to have electric start. The Robin went for sale in 1993 at 4,500 pounds, almost double what the standard Bullet 350 retailed for at the time in the UK. High cost, lack of performance and the unfamiliar look of the engine have been cited by some as reasons for low sales for the model. Several dealers in the UK, Germany and other places have undertaken "private conversions" of various scales, some doing one-off bikes per request others attempting a more regular production. It would be too lengthy to mention them all here, you can easily google them. Some examples are here, here, here, here and lots here.
Particularly noteworthy is the attempt by Geoffrey Baker, the President of the Royal Enfield Association in the US, who in a few weeks will try to link Vancouver, Canada and Nogales, Mexico using 10 gallons of diesel on a Military Bullet with a modified Kipor 11hp Yanmar clone diesel engine.
Private conversions continue in India as well. Some of these entrepreneurial fellows offer their bikes on ebay, usually to Canadian, Belgian and other European markets. I don't exactly know how easy/impossible it is to import such bikes these days, given the stringent pollution regulations going up in most places, etc. Buyer beware...
Oh, and that V-Twin monster at the beginning of the post? More details here.
In preparing this post a good source was "Royal Enfield, The complete story" by Mick Walker.