if you build it, they will come.
an undercover offroad track was done many many years ago, and it didnt last long due to the electricity bill as it was done at night.
we are long due for an indoor facility, and although there is already one, its really only catered for walk-in patrons, not racers.
I'll be all in, offroad and onroad, myself and a few others race weekly without fail(yes sad life haha) but the track/s has to be A1/technical for your venture to be successful.