First hello and thanks for reading this new thread hope it's not in entirely the wrong place.
I have a 2005 350z regular model.
It has a very loud misfire, still drives but with big bangs when you gas.
It seems to be coming from passenger side, also seems the backfire seems to be coming through the air intake but I could be wrong, there has also been oil above the engine on inside of bonnet not loads but a little spray after driving with the misfire for ten miles, my mechanic came over and listened and pointed out there also seems to be an occasional light tapping sound towards the back of engine driver side, when you rev up obviously the backfire gets very audible, he states that the engine has been opened before and not properly put back, also he thinks it may or may not be serious, lets assume it's something like the inlet valve is burned, can you please tell me what is involved to get to this part to repair, he has said he wants to strip down the heads and check the rings also, obviously he will do tests etc, but can't do nothing for 3 weeks, I have faith in him as he has much experience, but not 350s.
I have 3 options, sell car as is before he touches it, wait and let him have a go at doing it, he is doing it for love and the labour cost is set price, very affordable, plus parts.
Or pay 1500-2000 for a new motor and he will fit it.
It's a low mileage car in average condition that also needs other bits like brake pads etc,
can valve and rings be done without removing whole engine? There is no smoke out of exhaust also Mot advisory said oil leak, I only got car recently.


Could this oil be from the oil filer cap or where, and could this bad misfire be something as little as coils, plugs etc, engine light is on, no fault codes yet but trying to get another scanner.

I know it's hard to get a diagnosis this way but If the car sells I would look for another used model, would you keep the car and get the engine sorted by a mechanic used to engine strip downs etc as this may well lead to a better than average motor.
Perhaps someone on here would advise of what better components could be used to improve power aswell as longevity, don't want to affect insurance costs tho.

Sorry I am unable to use jargon, but this is my first post,
Thx again guys