My local shop has done many Mid drive conversions to MTB for hunters
its woodlands / tree farm - timberlands around here.. Elk & Deer ...
motor assembly replaces crankset
cost is $1600.. for parts & labor ..
you may not have any significant elevation gain to cope with then a hub motor scheme may be adequate..
(71, I'm Still Pedaling on my own)
>> I'd warn the issue with an old bike conversion will be inadequate Brakes <<
Have not seen a 'Copenhagen Wheel' in person .. did see diagrams , the battery is small capacity
because it has to be inside the wheel..
& they seem to sell for
$1750 ..
...