check for a vacuum leak on the thin pipe going to the MAP sensor (left side of the engine bay has a 3 pin wire connector). the rubber elbows like to split. the pipe can also chafe and get a hole in it.
caused one of mine with the 1242cc FIRE to be a bit slow.
a dirty fuel filter can also cause it to be fine ticking over but not great on load.
... and a crank position sensor can also cause issues.. make sure it has no iron filings stuck to it.
also i dont think it matters that much but make sure the crank pully for the alternator belt is on in the correct orientation.. yes i know it can be bolted on many ways, but it does have a little locating dimple it is meant to sit in before being bolted up. dont bother undoing the alternator to slacken the belt, you dont need to.. just take the pulley off. i cant remember the last time i slackened the belt on a panda heh just so much more easy to take the 13mm bolts out of the pulley.