Over the years I've shot sports, both Indoors and out. A one lens setup is very restrictive. Lens selection is based on many factors; indoor lighting (type and amount of light), type of sport (fast, slow, team, etc.), rules of the facility regarding where you can position yourself, will flash use distract the athlete, will you be shooting through plexiglass, wire mesh or have an unobstructed field of view, are you at field level or in the stands, end use of the shots, etc.

If I was limited to one lens I'd select something along the lines of a 70-200 f2.8 and adjust the ISO as required. This range lens allows you to get reasonably close to the action, isolating individuals when desired or going a bit wider when necessary. Ideally I'd also carry a 28mm or 24mm as I like the look when shooting hockey.

A 50mm is neither wide enough nor long enough in my opinion (I no longer own a 50mm). If you have the run of the venue and you are shooting a gymnastics exhibition you might get away with it. It really does depend on the type of shots you want. I like working close and getting facial expressions where possible when shooting athletic events. If cost is a major consideration, look into factory refurbished lenses. Or, consider a slower lens with higher ISOs.

The most important advice I can give is to study the sport and learn to anticipate the action, be ready and take more shots than you think you need,