Tiger, We would need the model and type numbers of the Briggs so we can see what you're working on.
Since sounds like the oil was polluted with gas, you should do a complete oil change change and not just out part of the oil. If you don't the remaining oil will still have gasoline in it which will vaporize as the oil heats up and can cause problem of over-acceleration as the engine will suck the fumes back into the engine via the breather.
Also the diluted oil can cause the piston rod to seize the the crankshaft journal from aluminum transfer. Once this transfer starts it will continue until the rod snaps. It takes time to develop as there is some lubing still going on so initially no damage is done just happens the longer you run the diluted oil.
As for the float valve and seat. I am thinking that you would have an all metal valve and a rubber seat. If so at least install a new rubber seat but most kits as both. Also the seat can be installed backwards; the groove side goes in first.
Hopefully you know it will smoke for about 10-15 minutes as the oil in the muffler burns off. Just watch out for the fire trucks.
