IDI stands for InDirect Injection, like 6.2l engines in CUCV's. DI is Direct Injection, like almost every engine bigger than that.
Small/old designs used a small chamber connected to the main combustion chamber- the injector & glow plug went into the small chamber. The glow plug can heat that space effectively enough to help start the engine. The swirl/turbulence of the air getting forced into that chamber on the compression stroke helped atomize the fuel- important for WMO & other thick fuels. It also allowed injection timing earlier in the stroke with lower-pressure injectors than would otherwise be needed at the same compression ratio. They're less fuel efficient & have more challenges trying to meet modern emission spec than DI engines, as modeling that chamber & the passage leading to it is a PITA. Very small engines also have a big challenge, mostly from the high surface-area/volume ratio in a .3L compared to a 1.6L single-cylinder engine.
I'd imagine one could build one from parts, but it'd be much cheaper & faster to order a palletload delivered to Canada/Mexico & have someone lie like a rug about them. "Scrap steel for traction weight, honest Officer." That'd look amusing in the paper. "OLD junk collector caught importing NEW junk!"
On a related note, the same companies that sold Lister-oid engines now sell air compressors- with a simple plug in the fuel injector port. Add injection parts & proper cams & voila!
Given that many Lister-fans want to run their engines on pure biofuels, I'm not even certain that it's legal for the customs guys to stop them, just too expensive to prove in court that veggie-oil compression-ignition engines aren't covered under EPA "diesel-fueled" regs. There's also been discussion about whether the fed EPA even has the ability to regulate stationary engines, as they aren't crossing state lines while operating.
/Back on topic-
A lister-powered genset can be made pretty bulletproof & fixable with a wrench & a hammer. Downside is the external oiling requirements. Very fuel efficient at ~650rpm, too.