Changing the gasket isn't too difficult in itself, but if your meachanic doesn't know Alfa's have may have set the cam timing without using the correct locking tools.
It is impossible to set the timing correctly without them, and if he changed the variator as well this will make the timing even further out.
If he has done this and skimmed the head you may have problems with the pistons and valves colliding slightly, however this isn't likely as the valves would soon break and cause the engine not to run.
Also double check with him that he changed the variator, ask to see the old one.
There's 2 ways you could tell if the head was skimmed too much, the first is a simple visual check of your Timing belt tensioner, though not thorough to conclude with, if the pointer of the tensioner is lower than the 4mm doton the tensioner fork then the belt is loose, if it's higher then it's too tight and may be making the water pump or idler knock.
The second (exact way) is to clean the lower face of the head an check the deepest part of the cylinder roof, it should be deeper than 12.8mm (ideally 13mm +- 0.2mm)
I'm actually chasing this very problem on a 145 cloverleaf at this very moment, and though we've found duff tappets and worn out cams, the noise is still there at 2600 rpm, I'm thinking it could be knock sensor, but atm the car is in bits with the head down to components in my garage right now. This is geting frustrating but the sump comes off tomorrow or monday to inspect the bottom end, to be honestjust riping the engine apart in one go and checking every component out is far less time consuming and a lot cheaper in the long run.
But back to your car, this 'one man band' who is working your car, he has fited the cam belt by loosening the inlet pulley and rotating it clocwise before fiting the belt, he's not just painted marks on and thought thet'd line up after???
P.S. Not all 'one man bands' charge for thier learning curve like him. Don't give him a penny till it's sorted, I don't expect anything till the customers happy, as long as they supply the parts. After all, I do this for fun and am not out to rip folk off, unlike him by the sound of it.