DOHC uses two cams ... one acts on the intake valves, the other acts on the exhaust valves.
SOHC uses one cam ... one cam uses its lobes to control all exhaust and intake valves.
IM's are different and aren't interchangeable.
EM's are different and aren't interchangeable.
There's more differences, but these are some major ones.
It would basically be a waste to try and convert a D-series to a B-series.