This is what i would do if it was mine or a customers OBM.
1/look at the colour of the tip on the spark plugs, make sure that they are the correct s/plugs, and properly gapped.
2/I would remove the carbs, strip examine and clean, re-assemble as per factory spec.
3/when it was running make sure the timming was within the factory spec.
4/ synchronize the carbs.Dont tuck the obm right under have it straight up and dowm.
when it is tucked under it is running in arich condition
5/to sync the carbs, with the obm ticking over the screws that hold the rod that operates the throttle slacken so the engine sucks the butterflys shut then carefully re-tighten screws.
I would set all the carb idle mixture screws at one and a half out from lightly seated, and dont forget to check the oil pump lever is in the correct position.
Obviusly check compressions
On the coast guard obms that I used to service I fitted a transit bolt across the the transom brks so the leg couldnt be tucked right under.