Your report on what you find, with regard to the current exhaust tappet clearance, the engine's behaviour when you correct it, and the sealing status of the air filter housing, will be appreciated by our members, Andy.
The existing air filter housing may function correctly if you assemble it very, very carefully and check it regularly - unless it has been damaged by heavy-handed treatment by now. The current production Inteks have a redesigned air filter housing which looks a more practical design at least in their publicity pictures. I do not know whether it can be retrofitted to the older engines - that sounds like a good solution if it can be done. If you find the answer to the retrofitting question, please let us know.
