Re: 0.0 Is this a CRACK?!
I am no welder but if a good magnet sticks to the bike frame I'd think it had enough steel to weld. If not you have your answer.
If you just want to move the bike for $200 post it up on Craigs and point out it needs a frame repair to be safe. You may just get a hobbyist that can fix it himself and get a bargain but either way you are off the hook as you warned them and was honest about the danger without repair.
I build a lot of bikes on the $170-$220 Macargi beach cruiser frames, even a 49cc 4-stroke shifter on one, but I would never use one for a 99cc 4-stroke like you want to do.
Department stores don't carry bikes like Giant or Raleigh for a reason, people that shop for a good bike don't go to Wally World to find one and their allotted space for bikes is filled with the cheap ****.
That is just my observation in Phoenix, it could be different where you live.