So I downloaded the Tekkit Classic map as I thought I'd go see what I'd really built back then...I had a vague look for instructions but didn't find much, so here's what worked for me.

The first thing was to install the Technic launcher and get it to put the ".technic" folder somewhere convenient. Then install Tekkit Classic and find the "saves" folder.

After some experimentation I found that putting the "world" folder into "saves" gets you a playable overworld survival map. To get the nether working properly copy the "DIM-1" and "DIMDIM-1" folders from world_nether to world. I presume the same works for "The End" but I haven't got a portal to confirm it.

I'm guessing the reason the files are like that is that on the server the dimensions are each assigned a separate process so they look like three separate saves that need to be merged in order to make one single-player map?

I'd actually like to load the map as creative mode. I don't know how to flip it into creative so I just made a new creative map, then copied the "region" folders in. It's good enough for screenshots but probably a bit broken.

One thing I've noticed in both cases is that buildcraft waterproof pipe doesn't render. It seems to be recognised as "ghost blocks" that you can't walk through but not as pipe.

Another thing I'd like to do is kludge in the "WAILA" mod as I've got used to seeing what a block is.