Subaru Impreza Turbo Set Up

[RCA]

Hi all, not usually on this forum!!, My cousin has a 1993 WRX STI which has had loads done to it. i.e. exhaust/motech ecu/a.p. brakes, can't say much more as I haven't a clue what I am on about!!! Anyway recently the engine went so he had a new one fitted which was fine but did not run as well as the old one, no where near as quick and ran a little lumpy. Recently it has got much worse and seems to be badly over fuelling, on tick over it has a bad misfire which does not clear till about 3000 revs. When misfiring it also puts quite a lot of smoke out!!. Is this just a case of needing a good tuning/set up or something more???, If so where would you reccommend it to be done??, I know he rang GGR but they wanted £800 to set it up and he thought this was a bit steep!
cheers
Richard

[RCA]

Many thanks for the replies, have been very helpful