Why have I lost so much H.P. on low RPMs? almost wants to die when starting off. on 1994 Mazda B3000

I have just replaced Rotor, cap, plug wires, & plugs. Helped a little, but not enough.

Asked by for the 1994 Mazda B3000
Exhaust restriction maybe? Any rattling noise from under the truck when running?? Possible broken catalist in converter.
Not so much a ratel as an exhaust leak. It's between the headers and the cat.
Rubber hammer...tap cat...listen for rattle.
Maybe plugged cat caused the exhaust leak?