Could be loose timing chain, could be the insulation around the fuel injector harness has turned to chalk, could be fuel pump, could be vacuum leak, could be failed maf, could be the caps inside the ecu have ejected their goo, could be somebody made ill-considered mods or a bonehead "toon"..
If I had to pick things to check first it would be timing first (there's a procedure for this..) and injector harness second.
I'd be cautious trusting a mechanic who isn't looking at those things on a classic car, there's really no mysteries left on these things.