I know this is an old thread, but about 15 years ago I bought a core aerator attachment for my Sears front-bladed roto-tiller. It has 8 wheels with 3 coring tines each, staggered for smooth operation. It simply replaces the tilling blades on the two sides of the shaft, and is fully self propelled using the tilling clutch to start/stop motion. I use the aerator attachment a lot more frequently than the roto-tiller blades. I have to run the motor on slow idle, otherwise I have to sprint across my lawn.
As I recall it was not meant to go on my exact model of tiller, but it works fine. The blades extend a little wider than the tiller fender, but it does not affect its operation. The holes for the clevis pin line up perfectly with the holes in my tiller shaft. Although Sears is gone, I suspect the company that OEMed it for Sears is still around (I suspect B&T, MTD, or someone like that), so if you have a front-blade roto-tiller anyone could probably find a suitable attachment with some searching.