Reddit mentions of Black Ops BB Conversion Kit (American to Euro), Black

The conversations not ideal. But they are not exspensive and work great in the short term. the only reason I am going this way is to give me more time to research what I want for a frame.

Source

from my memory of a couple of rides on ffs bikes (and a you tube video of one a schwinn) you are looking at a one piece crank system. the outward facing cups are pressed into an unthreaded shell and the cones are held by the cranks and tightened by the threads on the non-drive side.

one piece cranks are pretty heavy but still found on low end bikes and cruisers, should be easy to find replacement cranks knocking around a coop or lbs. the bracket shell is referred to "american" and there are three piece crank systems for bmx set ups, probably not best solution because arm length and chain ring size. lastly there and adapters to fit english threaded brackets into american shells ( ie https://www.amazon.com/Black-Ops-Conversion-American-Euro/dp/B001G8TSPS )

lastly note that in replacing the ffs you will have to replace the rear gear cluster with a thread on freewheel, otherwise there will be no mechanism for coasting and who knows what will happen to the rear derailleur.