You can put a bmx crank on that frame, if you like. No mods or special adaptors; just need the right BB.
I can't see the drop outs on your bike, but if they are vertical, you'll need a method for keeping the chain tensioned, and it won't work with a coaster brake version if you're running a tensioner. If the dropouts are horizontal, you should be good to go without a tensioner, and a coaster will be fine, although you may need to respace the hub....