I really don't believe this bike has 650B wheels. 650A is much more likely, much more common, and doesn't have the advantages that make 650B so talked about. Specifically 650B has a wide variety of high end expensive rims and tires; 650A is available mostly as cheap low end rims and tires.

My vote is fix up the bike and pass it on. If you want to try 650B start with a good old road frame that is worth investing in a minimum of $150 for a new set of wheels plus almost as much for tires.