So I tracked down a couple lasers via used Discmans.
I have a few more on the way (different models), and I plan on reporting what models carry compatible lasers. I'm going to try and check the lasers in every model Discman released between 1988 and 1991 and get a chart going since it's become very apparent to me finding new optical assemblies at a decent price is no longer feasible in 2008. It seems buying the lasers (new) these drives require is almost as expensive as just buying a whole new drive.
The Discman D-34s I got had KSS-220a lasers. I had some question whether these were drop-in replacements for the KSS-162a, and they are not. Almost, though. The optical unit itself is the same, but the mechanics that connect to the driveshaft are different. Actually, the KSS-220a is designed not to use a driveshaft at all but is driven by gears.
It's easy enough to get around this since the mechanics are attached to the eye by only two screws. Remove the two screws, and swap out the gear carriage for the driveshaft glide used by the 162a and you just transformed your 220a into a 162a.
I replaced the dead 162a in my CD-ROM^2 today with one of the 220a's in this manner and so far it's working perfectly. I had to tweak the laser sensitivity pot a little bit since it was singing initially but I found the "sweet spot." Anyone "attempting this at home," don't forget to tweak that pot when you're all done. You can significantly decrease loading times & re-seeks and enhance the laser's ability to read CD-R media by optimizing that sensitivity. Also beware if you're doing this at home that the process is very tedious and time consuming. Not difficult, just laborious. It took me about 2 hours to remove the bad laser from the drive, remove the good laser from the Discman, and install the new laser into the CD-ROM^2. It took another 30 minutes playing with the sensitivity pot to find the sweet spot for optimum performance.