m20 to m50 engine swap?

ok i got a 91 520i SE with body damage.. and somebody has offered me a 90' 520i for penuts but is missing its head throttle body etc...
Now was wondering if the m50 engine from my later 520 would be an easy swap into older 520 and if so what will i need to change.. ie ecu engine loom etc etc?
being as i really want another 5 series and mine is just sitting there as spare parts!
thnx
darkz