As you have everythign apart, it would be quite easy to clean. You can buy some descaling solution from Cafetto, as well as some cleaning powder (used for backflushing). You can soak the parts in the descaling solution you make up as recommended and watch all the scale magically disappear!! Rinse thoroughly (just follow instructions on the product). Same for the cleaning solution, you can make up the solution using hot water and soak parts in it, and/or scrib with brush, its amazing how much old coffee stuff gets removed. Rinse.
They you can buy a new group seal and all should be dandy. I cant think of any other bits that might wear out, perhaps the boiler o-ring if its too old to reuse?
Also, do a search for a recent thread on an easy fix for leaky steam valves, using a bit of heat shrink.