G'day Sapper and Tyler,
you most certainly can get an 18"mulching disc. I have two 18" thumb latch powertorque mowers with them, one of which I posted a pic of - my red one. No mods needed. I've sold two 18" steel deck mulch and catch 2 stroke mowers.
For it to work properly, need the mulch plug on the rear to form a vortex to chop up the clippings and force them down into the ground. That's the problem, they weren't made during that era. If you're good with metal, you could perhaps fashion something that blocks the exit of the clippings and forms a circular chamber under the mower, which the mulch plug does, but such a modification would probably have to be permanently held on with rivets or screws. Not worth it IMO. Who knows though, maybe it'd work brilliantly with a thumb latch alloy base.
