There still may be harmonics at certain RPMs that you may want to dampen out, but the "harmonic balancer" will typically just be a very heavy, balanced pulley on any boxer.
If the manufacturer didn't put one on, it probably isn't needed, and may actually create issues.
IIRC the Corvair crank does have some particular RPM//harmonic that should be avoided, and I do believe they make dampers for those, but it's another "nice" thing to have and not usually required..
I once saw a T1 VW motor set up with a very lightweight flywheel ...and a 6 lb crank pulley. These aren't active dampers, just heavy pulleys... and the pulley end really only has a woodruff key to keep things straight and is prone to cracking there, no taper.
(Any hipo T1 will have a fully counterweighed crank with additional mass, so adding more to the weak snout is probably counterproductive IMHO)
Looked cool, but I'm skeptical it did more than that, and perhaps less.
They DO make V8 style dampers for T1 VW motors IIRC, but they are very seldom used, and probably even less often actually required.
I don't recall ever seeing a huge damper on a 911, the fan pulley is typically pretty small dia.
What's your application?