The leakage likely was from poor float valve seal, perhaps a bit of dirt compromising float valve seating, rectified by your cleaning.
If bike runs with substitute carb, you've gotten a real heads-up on where to start.
I assume you disassembled the carb fully, cleaning all jets before re-assembly, float height correct, and no punctured float.
Recommend you check your diaphragm for proper sealing and any tears or holes; easy to get an air leak if the diaphragm isn't totally snugged in its groove around the carburetor mixture chamber circumference. Diaphragm air leak results in fuel-lean mixture at open throttle.
With suction on petcock vacuum hose, assume petcock opens. Assume "choke" (starting enricher) connected properly and functioning.
Assume spark; good spark plug and high-voltage wiring.
Sometimes, starting fluid can be your friend; will it start and run on starting fluid (have fire extinguisher handy)? If so, but not on gasoline, fuel system/carburetor problem likely.
Recommend jumping with a known hot battery (e.g., from automobile) and cranking with different choke/throttle configurations; even . . . with starting fluid, for diagnostic purposes.
I can't imagine how welding on the chassis or anything connected could cause a problem with starting; probably coincidental only, IMHO.