Misfire caddyvan 1.9sdi

Misfire caddyvan 1.9sdi
Hi looking at van van to buy but it has a miss fire the bloke has a spare diesel pump and say it misfired since the belts was Done what could be the problem and could it be a serious problem

Got told the pump could just be timing slittly out

hi fella,how many miles has the engine done?,normally if the belt has been put on wromg the first sign its s**t mpg not a miss fire,unless its more than a tooth out,or it may be the belt is more than a tooth out so the valve clearance are out so the valve are not closeing fully but in this case it would run rough on tick over on all 4 pots not just one