on some no tolerance engines, if the belt goes, it usually bends valves and can damage pistons or other. If the head was repaired/replaced and the block components are ok, there is no reason whatsoever that a big job like that would not restore the engine to it's proper performance. I can only say that the most likely cause would be improper intallation of either timing components, or peripheral compnents dislocated from the head or possibly collapsed lifter(s) if break-in was not precise. The odds of a non-related failure coincidental at this particular time is very very slim.
I would hope they replaced all exposed components in timing area. pulleys, tensioner, sprockets, any timing belt driven pumps etc...
It could also be that something was overlooked. If they did the timing properly, I would hope it would be wither a ignition, fuel, emission component problem such as forgetting to plug in a harness connector or actually breaking a sensor, voltage spike to PCM, snapping plastic vacuum hose or misrouting a solenoid or other vacuum activated component.
If they don't fix it properly, document everything and write your State Attorney General with the complaint.