The main jet will be stamped with the number 78. You can see it with a magnifier without removing the jet.
The throttle plate has the number 11.5 stamped on it. Measuring the venturi dimeter with internal calipers confirms it.
There's no reason i can imagine for replacing the carb.. and anyway i'd do whatever possible to repair and keep that original carb (or replace it with an identical one). It's a great, simple carb and, like i said earlier, is capable of fueling that bike up to near 40 MPH without running lean. Performance at lower speeds is trouble free. In conjunction with the reed-valve intake porting a nice, slow smooth idle is to be expected.
Honda knows how to build bikes. I have three PA50 IIs. It's a nice bike in stock condition with lots of hidden potential for more speed just the way they made it.
The carb is hard to access but i still made the mistake of hurrying the carb cleaning job more than once. The result was always the same.. Drop the engine and do it over.
Remove the main jet, push or blow the internal brass emulsion tube down and out and clean up it's cavity above the jet.. REmove the screw that hides the idle jet (its the one below / next to the idle speed screw) All the passages and parts should be soaked and blown out with strong compressed air and examined closely. The bowl must be immaculate. Any old fuel residue on internal parts will come loose and clog something.
Coat carb/intake red valve block mating surfaces with Permatex 2 (non-hardening gasket sealant) to prevent vacuum leaks since the old gaskets and o-rings in the area tend to develop vacuum leaks. Speaking from experience, don't use sealant on the float bowl O-ring because it will squeeze out into the bowl and end up clogging the main jet.
Old Hobbits with old fuel systems initially need extra effort, but once cleaned up and kept clean they are trouble free and will run forever.