When I bought my 83.5 Westy I did not have the key.  Your local locksmith
can make a new key.  It was not expensive.  I would have to go back in my
records, but it was probably no more than $10.

You need to take out the assembly with the four screws and take the unit in.
I don't remember if I removed the lock assembly from the plastic frame or
not.

- Mark Rissman
83.5 Westy