Make sure they didn't pinch the 02 sensor wire between the motor and bell housing, I have seen that before (don't ask) and look for damage to the wire (maybe check for continuity) They may have pinched the wire, done up the bolts... found the wire... undid the bolts and free'd the wire, but it might be damaged internally. Check they didn't forget to connect any earth wire/strap from the block to the body or battery.... the exhaust being insulated by gaskets and rubber hangers might not be connecting to the ground properly? Does your 02 sensor have one wire or 2 (or more)?
Disconnect the battery and un plug all of the connectors to the engine, inspect pins in connectors and re plug in clean with some electrical grease.... un plug everything and re connect it.... you are looking for any pins that have maybe 'fallen over' .... if any plugs unplug too easy, try gently bending the pins so they make better contact..... unbolt the coils and clean the bolt/earth points with sandpaper so it is getting a good earth.
Also, check they have earthed everything, connected all vacuum hoses for leaks (stick a bit of garden hose in your ear and poke the end around potential vac leaks while engine running), check the nipples on the end of the spark plugs are tight.... I have had a miss in an engine and found the spark plug nipples were loose.
Before reconnecting the battery, may as well do a computer reset....
I would also be getting the mechanic to sort it first.... before doing too much.... if you change a whole lot of stuff, he will just say you have been playing with it and stuffed something.... but to me, if it went in running good (but over heating) and they have done the timing right, it has to be a small peripheral that has been left un plugged or connected badly. Run the car at night and look for plug wires sparking/leaking on something metal
