You can use the original motor wheel itself, with the motor--but you have to take the wheel apart on one side to take the hub apart to get the old controller out, and add a cable from the phase and hall wires inside to the outside, to run via an external controller.
There's a few Stromer threads around the forum with some info on that:
https://endless-sphere.com/forums/search.php?keywords=stromer*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
like these two
https://endless-sphere.com/forums/viewtopic.php?f=2&t=86600
https://endless-sphere.com/forums/viewtopic.php?f=6&t=63809
AFAIK, any regular BLDC controller can run the UltraMotor hubmotors in these things; just use one appropriate to the motor wattage you've got, and the battery capability youve got (so it doesn't draw more amps than your battery was designed to output, and doesn't overheat the motor).
If the battery you have is one of the ones that only turns on it's output for the original controller and LCD, then you'll either need to replace it's BMS with a "standard" one, or wire around it's output FETs (which will defeat it's ability to protect the battery from overdischarge).
If you can't do that, or don't want to, and/or you need more power than that battery can supply, you'd have to replace it with a different battery (external rackmount, for instance, or something in the triangle, or elsewhere; lots of options depending on what you need out of it).
The Stromer LCD won't work with whatever elseyou put on there, and neither will any PAS or torque sensing it came with, so those will have to be replaced with ones that come with the new controller.
If you literally just want a motor/controller/display kit that will work, it's likely that any typical 500w rear hubmotor kit will fit on there, though you may need spacers to make the wheel fit without bending the frame in--I think the stromer motor's axle shoulders are wider than the common 135mm bike dropouts most rear hubs were made to fit in--something more like 140-150mm, IIRC. It's in my thread linked above.
A fatbike hubmotor might fit in it's place, but the fatbike *rim* probably won't, so you'd likely have to relace the fatbike motor into a rim that will fit on this frame. (like the orignal off the old motor; you'd just need to measure it and the new motor to get new spokes, like here at the https://www.ebikes.ca/tools/spoke-calc.html page).
