50cc Gas Pocket Bike Inquiry

I'm considering buying a 50cc gas pocket bike that's been customized a little bit. I was going to buy a new one but I don't feel like spending the extra money when it's not necessary.

The one I'm looking at has a broken pull starter and will need a replacement. Does anyone know about replacing these? Or how much I'll be spending if I get it replaced. I'd like to do it myself but I've never owned one or anything like it, so I wouldn't know where to start. If it's simple I'll be able to do it, if not, who knows.

Any insight is appreciated.