[Uhehner]

I can only suggest that you would start by drilling some holes in inconspicuous places and inserting a fiber optic line to survey the area. Then layout your cut lines with tape, set your circular saw depth and hope. Maybe screw a support block to the deck after you make each cut to prevent the floor from dropping down.
Give yourself plenty of clearance, better to cut out one large section than lots of small sections. Good luck. Post your results. Capt. George