a few points. 2.5 and 3.0 have a dispac at the rear of the engine and can not be bathed in oil. 3.2 and 2.6 have coil per plug set up and can be bathed in oil.
If you have leads then you have seperate dis pac at the rear. So no oil but they do split and allow the spark to escape.
If you have changed spark plugs and leads and your sure its still miss firing then it only leaves the dis pac to change, if your sure the plug leads are connected correctly (breakers on here can help or there is a guy in Heathrow who specialises in breaking omegas).
Exhaust unlikely to cause a missfire.
If hes done your cam covers make sure the breathers are cleaned or they will just blow again as its the breathers that cause this. Guide in maintenance for cleaning breathers. Print it off and show him, or diy. Its fairly easy.
The original dis pac is made by bosch, if you want new you could contact them direct or through a motor factors. Such as Camberly Auto factors or similar. Hth