Complex sentence: Today the circuit does not host such high-prestige events as facilities are no longer adequate; the circuit would need some investment in order to get back onto the BTCC and F3 calendars.
Split into simpler sentences: Today the circuit does not host such high-prestige events. The facilities currently at the circuit are no longer suitable for hosting such events and the circuit would need some investment in order to get back onto the BTCC and F3 calendars.