Seals are a good suggestion, it is possible to change out the flywheel side fairly easily but changing the PTO side is most easily accomplished by splitting the crankcase. Not really a big job but does take a bit of time and clean up.
Updating the carburetor is not really an option without changing the tank bottom/air box; in fact McCulloch offered a retrofit kit to achieve that task to provide the additional clearance needed for the taller cube style carburetor.
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Your carburetor is indeed the single needle type.
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The low mix screw (red circle) should be 2 to 2-1/2 turns open from lighted seated, hold your finger on the butterfly to determine when the needle is seated.
The idle speed screw (green square) is used to control the idle speed, some have a low speed governor as well.
The high speed/mix screw (blue arrow) controls the full open position of the throttle (+/- 7 degrees of horizontal won't affect the throttle opening) but controls how far the needle is "extracted" more extraction for a richer "H", less extraction for a leaner "H".
Easy as that.
The duckbill valves are NLA but you can get by drilling out the passage to 5/32" and using the red Homelite duckbill.
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Mark
