Give the agency the required notice then leave that agency.
Start your old/current job with the company/new agency.
If the old agency get in touch and moan, tell them you'll see them in court.
Trust me, it will never get that far. The money it will cost them to bring a civil case against you will be far more than they could ever recover through any finding against you. It's Simple economics really.
That clause is in your contract because there are many pilots out there who will sign up to such draconian conditions and is designed to "intimidate" you into not leaving that agency to go with another who offer more favourable terms.