On a budget, I would:
Buy 3 treated 100x50 C16/regularized timbers min 1m longer than the boat and use them to support under the tubes and the third under the keel, make metal brackets to the existing trailer as needed. Bolt or weld on and coach bolts to the timber, cover the three bunks in cheap industrial cord carpet. Remember a metal tie down/winch post at the front center.
With the off cuts of timber: make a profiled piece that fits under the transom and takes all the weight of the motor, cover in carpet, fix to trailer.
I did this to a unsuitable trailer that came with a SIB some years ago, its worked out great and gives good support, 400 mile round trips have been no problem.