The problem I've had with metro's is the pre-cat located in the exhaust manifold itself. If you don't live in California, you don't even need this piece of junk. It overheats the heads and makes it crack, or makes the head gasket leak. Removal is rather easy though. Remove the exhaust manifold and chip that baby out, use a shop vac to remove the pieces. If your model don't have one, it will usually be the one on the exhaust pipe itself. But you already replaced that. Try a short test run with the manifold disconnected and see if acceleration improves to rule this out as a potential problem. Also check your TPS, (Throttle Posotion Switch) with an ohm meter to be sure it hasnt developed any dead spots. Yet another source of trouble seems to be a stuck EGR (Exhaust Gas Recirculator) valve. This will allow too much exhaust to enter your fresh fuel air mixture, decreasing detonation, and slowing acceleration.

Timing Belt