Can i turn a mandolin into a ukulele?

Probably not. Ukuleles and Mandolins have a completely different sound and I doubt the nylon strings on a Mandolin wouldn't sound very good at all. You could buy a cheap yet solid Ukulele for under $60. Plus, vintage mandolins can be extremely valuable. Maybe do some research or ask an expert on it and you can make a lot of money if you get it fixed up.