terzo engine issue

[jimtech]

Hi all. Im a bit new to subaru's and have an issue with a terzo which is on a 1998 on an R, Some times there is a drop off in power and a misfire. which is intermittant. The engine has been quite modified and am unsure of what.The car belongs to the father in law and took it to a mate who had a diagnostic tool and came up as the knock sensor. Im a mechanic myself and would possibly put this fault down to it having the misfire now and then. Are there and common issues with misfires on these turbo engines so I have a bit of a heads up before I have a good look at the weekend?
Any help would be great