MS4B Would be an original 1929 carb. number and known to be the one that crumbles. I also have a complete survivor but won’t use it for that reason. The numbers you’re looking for are the MS-45A & MS-45B, used on the Neptune OB-2, OB-3, OB-4, OB-5, OB-31 OB-32, OB-51, OB-61 & OB-63 from 1929-35. BUT you want one from the later models as the early carbs have the same issue. I believe the MS-45C, & D, from 1936-37 are also the same carburetor. This model is made up of parts from other models. From a failing memory I believe only 3 parts were manufactured to make this motor. The drive shaft from the crankcase to the impeller. The muffler. It looks the same as the N but its not. And the tiller assembly that connects the power head to the folding assembly. These motors are not rare but they are uncommon. Here is a link to a video I did on these a couple years ago. https://youtu.be/i9NiPC126fY?si=Y2HmmgQTwk4ooo01
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.