The most likely cause for a chain to brake after a head gasket job on an M103 is that the chain tensioner wasn't re-set before it was put back in. When removed, it t must be pushed all the way through the click barrel and then put back in the front again to reset it. It will then find the proper tension via the spring when the cap is put back on. If you don't, and just put it back in the way it came out, it will be at a setting that can over tighten the chain and snap it within a few miles.
Also, as mentioned above, I believe this is an interference engine, meaning, that if the chain breaks while the engine is turning, the pistons will hit the open and out of time valves causing extensive damage to the valves and pistons.