You will want to implement some form of game states. Your race/main state will setup all of the models, inputs, etc. You will want to parent everything to a node control by the state instead of directly to base.render. This makes cleanup easy since you can just remove that node and everything under it. Upon exiting the state you clean everything up. Now, to reset a race, you can just destroy and recreate the state.
You can find some old, undocumented code showing an example of game states here. In particular, I think you’ll be interested in the CombatState