My carb ...Nikki # 799255 needs replacing...anyone know what to replace it with other than an expensive OEM one? Tried rebuilding and the plate that the main jet sits in will no longer hold the jet in place. TIA

Is the o-ring still good on the main jet (to hold it in place?
 
https://www.ereplacementparts.com/jetmain-standard-p-227614.html 
 
 

Briggs does show another carb option along with the Nikki:
https://jackssmallengines.com/jacks-parts-lookup/manufacturer/briggs-stratton/briggs-stratton-engine/200000-399999-series/21r700-to-21r799/21r707-0069-f1/carburetor-ruixing 

Seems to be round and pliable enough, no cracks. The bowl had a lot of "crud" in it. I throughly cleaned it , got back togeather then found out jet will not stay in place also appears now that the float needle is not seating so bowl is spitting gas back through the air intake.
Think I am just going with a new carb...just trying to fig out what to replace it with.

Thoughts?

I swapped out my leaking Nikki on my 18HP Briggs (Toro zero turn) about 4 years ago
when I re-built the engine. 
 
The Nikki had (0r was supposed to) have a separate o-ring, (slightly different than yours) that went in between the white plastic part and aluminum body.  That leaked fuel directly down the carb body.
 
I've since repaired that carb but at the time just put the Ruixing on it. 
 
The Nikki carb is a real crappy design, the Ruixing, pretty much common (just one outside edge o-ring).
 
I have no complaint's with it..
 
.

So do you know the part no. for the Ruixing carb I will need?

This one should work for you:
 
https://www.jackssmallengines.com/jacks-parts-lookup/part/briggs-stratton/594593 
 
You may want to call Jacks and confirm before buying