I'm not familiar with all the different brands of tillers, however i'm somewhat experienced with chain drives on farm equipment.
It looks like a broken chain tensioner. The D portion fits in the slotted fitting, and the curved portion (It should be all one piece by the way) is adjusted to take most of the slop out of the chain. There should be a pin or perhaps a set screw to hold the D portion of the adjuster in the slot. It needs to be held in by some method at least.
I'd guess theres a nut on the back side of the slotted bolt that you can loosen and adjust the curved portion of the part to run against the chain to take up the slack.
Rather an unusual adjuster that i've never run across before, i've never seen a metal cam such as this for an adjuster. Usually they will be an idler gear or a wooden block to help prevent excessive wear.
It looks like, lacking a parts book, you'll have to have the two pieces welded back together and determine how its held in the slotted bolt to allow adjustment.