Lens recommendation

I am looking for suggestions for a lens to use with my OM-D camera for shooting indoor martial arts photography.   My son has a belt test in March and I am looking for a decent lens in the $300 to $450 price range.

The camera body is OM-D and I have the 14-42 and  40-150 mm kit lenses.     My son will be anywhere from 15 to 40 ft away as it is a small dojo with two walls of the dojo having 2 windows letting some of the outside light in.   I have unrestricted access to move around the edges of the room while taking photos.    Would the Olympus M.Zuiko Digital 45mm 1:1.8 lens be appropriate or should I use a longer focal length?  I understand that this is primarily a portrat lens, however would it work for indoor sports if the distance to my subject isn't too far?   Also looking for other lens suggestions.

I have had my OM-D camera for a couple of months and am a newbie to photography.

Thanks,