Ruger will fix it, or Brownell's will have the parts. Given shipping costs on handguns, Brownell's will actually be cheaper, especially if the hammer, trigger and transfer bar group are intact.
Lesse...here's the list of Rugers:
http://www.brownells.com/aspx/NS/schematics/SchemMFG.aspx
...but we got a problem - your gun isn't listed. Durn.
OK. Betcha the firing pin/spring assembly is used in several guns. Let's check.
Redhawk: whoops, not available for sale. GP100, same, and according to the pic it's a different shape. Uhoh.
SP101, they do sell the bits.
Odd.
You might consider going to
http://www.rugerforum.com under the gunsmithing forum, see if anybody knows if the old Speed/Single DA series is compatible with either SP101 or New Model Blackhawk bits will fit.
If you can't get an answer, it'll be off to Ruger.