You can run this game in 2K without any upscaling! You'll have to use dgVoodoo2 and force resolution to your screen size (in 4:3 of course). Stretching mode should be set to “Stretch, keep 4:3 aspect ratio” (or similar).
The result is a FULLY UHD LEGO ISLAND!!! Of course, one catch: It seems that you can't use this with LEGO Island Rebuilder. Which is probably a massive catch, now that I think of it…