rattly sprinter engine?

i have a 1995 merc 208d sprinter that has been very rattly after a head gasket repair it is also belching black smoke when getting up the rev range.it is also very sluggish. i can find no apparent cause for this as every thing is timed up correctly and all the valve train is in good order. also this morning it has made some horrid noises and now refuses to start but nothing appears to have failed. please help i'm at my wits end with the wretched thing