Typically, that's the seat switch. Either bad or out of adjustment. Make sure the connector is fully seated etc.
Without a schematic, there's no guarantee that's the problem however.

Is there a model number on it similar to this format?
13BT604G401

I would go over the routing of the belt first and make sure didn't bump any switch connector off, Changing the oil, air filter, fuel filter, batt. and fuel lines shouldn't stall the engine out unless the "fuel system " still has "air in the hoses from changing the lines" especially if it has a fuel pump on it (usually gravity feed system (tank mounted above the engine) is not as touchy as tank mounted below the engine.

I would run it a while at different throttle settings to make sure all the air is worked out of the carb and fuel lines. I still not sure what you mean by stalling out. Does it die??? or does the RPMs come way down then go up and down???

I was having the same problem with my John Deere L130, checking every wire and switch for a good part of the last two weeks, until I saw this post. Sure enough, it was the reverse safety button.

murrey 1973 lawn tractor mint shape , 38 inch cut , runs great till you engage the baldes then stalls

My 2000 craftsman riding lawn stalls out when blade is engaged

So what other problem besides the reverse switch and the seat switch could cause the engine to cut off when engaging the blade?

could be the cl/brake sw, but depends on make/model..