Spark- How is mag timing? Has the 500 hr mag inspections been performed?
Fuel- Any difference with & without boost pump on? Checked fuel primer lines and connections?
Carb- overhaul cost shouldn't be factored in, get it done!
One rare but possible cause- There is an old AD out on Marvel MA-4 carbs dealing with the lean mixture valve rotating in the bottom of the fuel bowl, although not applying to clone carbs, can happen as it did to me on a O-360. Depending on the valve movement in the fuel bowl housing, would either cause an over lean or over rich condition depending of throttle set & would either starve or flood the engine. The clone carb on my engine was a new AStar unit.
Maintain lots, upgraded - repaired - & modified more, rebuild a few, & built some too.