There are many, many possibilities, but the engine needs a good battery, fuel delivery, compression and spark to start and run. You are probably missing one of those.
How do you know that it is your car?
It wont start
Your car wont start. lol
use jumpers
What have you welded ?
how does it stall if it wont start?
catalytic converter?
car wont start
Don’t no
Maybe Its The Muffler or the pistons hitting the engine
You may have overheated the engine or seized it.
lack of fuel