Fabricate the winch post as per the diagram below, and using your winch base as a template, drill the mounting plate to suit.
Check that you can fit the bolts into and through the winch mounting plate once fitted to the winch post and if there is interference, make the necessary adjustments. If required, the winch can be mounted parallel to the drawbar to assist with fitting the mounting bolts.
Check out your local chandlery or boating supply shop for a suitable diameter pulley (50-80 mm, but smaller will work at a pinch) ideally fitted with bearings (the pulley quality will determine how smoothly and efficiently the trailer will tilt). Adapt the pulley plate to suit both the clearance between the pulley and the pivot arm (enough to thread the winch rope and hoop through ) and the pulley hole diameter.
The safety plates are for additional security during the trailers normal use should the over centre latch or winch fail. Shackles, karabiners, twist locks or anything similar can be used to secure the safety plates together.