Actually don't know where to start with this one. Generally there is a logical approach to fault finding in that from experience A generally fails this way, which means it's likely B is affected (is it yes/no?) and gives X, Y and Z symptoms.

If someone has gone in and modifed a load of stuff to varying quality then all of a sudden that thinking doesn't apply anymore.

Anyway. I guess if it has had a de-cat and an intercooler, then it has certainly been remapped to goodness knows what. Your high Co2 could be related to engine overfueling or poor fuel/air mix from the remap depending on what the customer wanted (pops 'n' bangs, innit) or the cat missing or broken or dirty or indeed missing MAF & Lambda sensors.

The high mox reading is likely beccause it doesn't have a cat.

All the others are mostly easy fixes that won't cost you too much at all with the exception of the, missing cat , oil leakand subframe corrosion, the latter two of which arn't a fail yet.