For a while, I was obtaining HP-42S calculators via the following subterfuge: I bought any "broken" HP-42S I could find. I'd also obtain another relatively new Pioneer (usually a business model) at a bargain-basement price. (Nobody wants an algebraic HP . . .)
Using the techniques detailed in "Pioneer Repairs" (under "Repairs & Batteries" on the MoHPC main page -- http://www.hpmuseum.org/guest/brogpion.htm ), I swapped the guts of the two calculators. (You might need to cut a hole for the infrared LED.)
I then sent in the "broken" 42S case (with algebraic guts, and maybe even a 7-segment LCD from a -20S!) to HP, got a new HP-42S service replacement for $70, and auctioned that "Mint In Plastic" on Ebay for (typically) $270. (But this is all moot, as HP's service of the -42S is presumably no longer available.)
However, the -42S guts usually worked with the newer (not worn out) case & keys. (If you need a new LCD, use an HP-17B or BII for replacement.) I spray-painted the keyboards a nice, designer-ish flat black, and taped a flip-out, laminated color print of the MoHPC image of the HP-42S to help with the keyboard recognition. (Another form of creative use of MoHPC imagery -- thanks again, Dave!)
The point being: believe it or not, repair or replacement of the keyboard and/or display components of a much-loved Pioneer is not impossible. If it's the mechanical or display parts that are broken, your HP-42S is not beyond hope.