Definitely not an easy repair. If I were doing it, I would make a new eccentric link, drill the tiny holes in it, try to save the existing rivets and reuse them to put it all back together, and if they can't be saved, use those natty brass rivets I found at Scalehardeware.com. Without those, some brass wire smacked on an anvil to form a new rivet.
Any way you slice it, it is not an easy thing to fix. You could try to remove the screw and solder the tiny broken piece back on, but that will also be very tedious.
I'd look on eBay for a dead engine that has similar links, or send it back to Bachmann for repair.