he is not talking about the ABS sensors.
cutting a hole on the drivers side makes changes to the cars chassis and it's crumple zone/design.
which also means it effect airbag deployment etc.
your car is currently illegal. - you already know this.
patching up the hole (really well so it can't be obvious that there way a hole there), should be fine in terms of the law.
also, any exhaust shop that has the tools and ability to cut/weld aluminium can re-do the current coolers pipework.
but if it were me, I'd sell it, patch the hole use your stock parts and an R34 GTT intercooler.
R33boy - I can't comment on if it is better or not. just that it keeps your engine looking stock.
If I were to go that path, I'd paint the cooler and the pipes it comes with Black also.
But I'm boring like that.
It looks good in the picture, but some guy on these forums fitted on to his R33.. and either he really bodged it up, or the kit lines up really badly.
maybe search the forums for "cooling+pro+stealth" or something..
see his pics.
or was that you? lol....