It's not impossible to repair it, if you have the time, and the cash that is...

You need to remove the metal bottom cover.
Unplug the cables from the main circuit board (make a note of what goes where), and remove the main board.
The whole keyboard assembly should them be accessable for removal.

The keys should still be available from Roland, and it might be worth replacing the key contact strips too, again these should be available from Roland.

Another option.. Find another D-50 for spares or keep the one you already have for spares..