Using crankcase pressure to power a Walbro or Zama pump has its problems.
The crankcase vent cannot be hooked directly to the carb as you still need to vent the crankcase. The solution is to put a tee in the vent line with a line going to the carb.
There's another problem as the diaphragm area eventually fills up with crankcase residual oil and the pump action gets weak.
If you are going to use a two stroke carb . . .
A four stroke 180 needs a carb that was suitable for a 90 two stroke as the four stroke only fires half as many times.