I had a lawn tractor with the nikki carb, had a lot of trouble getting the bowl gasket to seal whenever I pulled it to free up the float and needle. I looked at the parts list for the motor and saw that there was also a walbro carb listed for that model. Ordered the carb and it has not given me any grief since (other than the occasional cleanout). In fact, when the engine started rapping and burning oil, I moved it over to the replacement and it is still going. It's not that I think the nikki's are bad units, I just think the walbros are less fussy and precise and cope better with fuel that isn't perfectly clean...and I can actually work on them.
I would be very tempted to find other similar size briggs engines and see if you can find a different brand of carb that will fit. If not, I would still be tempted to just get a new carb, even a nikki. I have been told two different things by small engine techs I have talked to: that it can take days of soaking and carb dip, poking at holes, and blowing out with air to get some small engine carbs clean; and also that sometimes the combination of water and ethanol in the gas can leach zinc out of the diecast and make it slightly porous leading to leaks of gas or vacuum. A new carb would at least let you start over with either issue...?
I would be very tempted to find other similar size briggs engines and see if you can find a different brand of carb that will fit. If not, I would still be tempted to just get a new carb, even a nikki. I have been told two different things by small engine techs I have talked to: that it can take days of soaking and carb dip, poking at holes, and blowing out with air to get some small engine carbs clean; and also that sometimes the combination of water and ethanol in the gas can leach zinc out of the diecast and make it slightly porous leading to leaks of gas or vacuum. A new carb would at least let you start over with either issue...?