First step tuning would be to set the valve clearances - 2thou inlet, 3thou exhaust. Clean the carb and set the air gap on the flywheel/coil 8thou. There is an article on the tech section of this site on valves settings.
You said it was box gx160 - has it been degoverned (is there an arm poking out of the top of the crankcase?).
Check the compression - might need valves lapping and/or rings replaces and bore honed.
there is loads of infirmation on the GX160 engine as Kewy says on the 4 stroke section of this site but if you want some real insight and understanding of the engine Brian Pollard produces some very good ebooks that will be worth a small investment. ukpollards.co.uk
Graeme
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