You have identified 1 problem (overfill), the oil in the air filter housing could be as simple as having been picked up from the crankcase ventilation system if it is laid out like the VW Rabbit used to be (1976 vintage). Without being an Iltis expert, I suggest fixing the overfill and cleaning the housing first, possibly a test drive then doing diagnostics for compression or leakdown if a problem remains before spending money (and time) on anything else. Take a look at the spark plugs, they may have gotten oiled and need either cleaning or replacement. Also, if the oil pool in the air filter housing was ahead of the filter I suggest a new filter as it may have become oiled and will either restict flow now or collect dirt very quickly once oiled and clog.
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