Nothing you've described sounds like a real cue to rebuild the engine.

Oily plugs - does it burn enough oil to see it coming out the tailpipe? Do you have to add a qurt more oftern than every 400(ish) miles? Does it just leak oil and not burn it? In general terms, oily smoke out the tailpipe under power is more likely work rings, but oil smoke out the tailipe only on decceleration is likely just valve stem seals.

Turning the engine by the fan often results in a slipping belt - there is just a fair amount of friction inside the engine (all four pistons sliding the rings up and down) and the belt isn't meant to handle that. If it was tight enough to turn the motor with it it would probably wear out the water pump bearings quickly.

Rough idle can be any number of things - maladjusted carbs, loose/tight vavles, etc. Before you get into things like leaking valves, worn out rings, etc.
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I'm JohnMc, and I approved this message.