You don't get the key apart. I understood you don't even have a key...
You need to get the green Volvo manual(s) which tell you about everything
you need to know.
A locksmith can make you a key but he will have to have access to the car
or at least the ignition lock to do so. Once he does that you can remove
the lock from the car fairly easily. Two screws go in from the bottom of
the dash to mount it and after you get it out, you can see a hole in one
side. With the key turned on, there is a button you can push through the
hole that releases the lock from the switch.

Parts that I am SURE interchange include the engine, transmission,
power train, radiator, wheels, hubs, front and rear suspension.
I'm fairly sure that there are many more that are directly interchangeable
or adaptible. Since I have never owned an 1800 and only worked on a few,
others can answer this much better than I.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma