those cranks are cottered, so if you aren't gonna use them you'll need a new bottom bracket. probably wouldn't be a problem. also remember you'll need a pretty high ratio for those tiny wheels.

as for what to do, you'll need to relace the rear hub to a track hub. that's about it. check spacing, I don't know if folders use the more or less standard 120 spaced rear. also, it may be hard to find spokes that short. that would probably be the largest difficulty in this build. (well at least with that blue one. that raleigh looks like it has short but pretty available spokes. the blue one has teeny tiny wheels lol)