I assume when you say valve cover you mean the dome-shaped nut which holds the ball-and-seats assemly in place. It would be a shame to have to install a new valve just because the seats are bad, but I understand the problem with the "crust" . Try spraying the dome nut with vinegar or CLR. Allow to soak, and repeat this process every hour for as long as you have the patience. Finally, allow to set overnight and see if you have any luck. If you can get it apart, all the internal and external components are readily available as replacements.
Replacing a body through the access hole in the tile or fibreglas is not fun. Not impossible, but probably also not a job for an amateur.