Good to see you working on it instead of selling it
I would put a 'positive' bevel on the existing deck and a negative bevel on the new deck pieces. Then attach cleats to the underside of the existing deck. This will allow you to screw the new deck to something and the top of all the pieces will be level. When you get ready to lay the deck down, butter the beveled surface with thickened resin and screw it down that way the pieces are already bonded before you lay matt over the deck.