The first thing I would do is pull off the silencer (little muffler on the end) and run it... any difference?
Then I would buy a brand new spark plug and run that... any difference?
If neither of those work >>
Then I would pull off the gas line to the carb and make sure it flows good.... good flow?
Then I would take the carb off and clean it correctly....and run it...... still bad?
Then I would clean and set the ignition points correctly...(if you have points.. I think it does)
You did not harm anything by welding a crack in the pipe...(unless you stepped on the pipe and crushed it to half its diameter... and I'm sure you didn't)
About 95% of all moped problems are
dirty, misadjusted points
old spark plug
The odds of your problem being the pipe is about 1000 to 1 ...(unless the silencer is clogged)