Things to check:
A. Size of bottom bracket shell (the tube that's part of the bike frame, that holds the BB bearing & crank spindle).
B. The type of spindle needed for your chosen cranks.
C. Possible need for an adapter to fit Spindle type B into frame size A.
If the bikes currently has a 1-piece (Ashtabula) crank, it likely has a large "US" size BMX bottom bracket shell (abut 2 inches diameter and no threads in the frame). It has to be that big to thread the dog-legged 1-piece crank through the frame.
That's good, because it's big enough to fit (almost) any other type crank spindle (possible with an adapter, tho), such as a "Euro" size MTB BB cartridge etc.
Your chosen cranks: Do they use a "square" spindle, or the classic "Profile" type BMX spindle (typically 48 small splines), or something else?
(Almost) All 3-piece cranks will be made for 9/16 pedal spindles to fit your DMRs, whereas you've already found that the 1-pc cranks can have 8/16 pedal threads.
Sheldon is your friend:
Pics here: BMX size BB shell with an adapter to fit a road/MTB sqare taper BB spindle; later fitted with a Profile BMX crank with a splined BMX axle in "US" BMX size BB bearings (no adapter needed).
Hope that helps for now.