As one who is pretty familiar with those boats, and their penchant for rotten bulkheads as the result of leaks
around the chainplates.... here's some advice...
DO NOT under any circumstances sail the boat. You will have mast in the water
at the first tack in any wind greater than a mild breeze. I know where you are sailing, have sailed it and you will have a disaster.
Get a bunch of jerry cans and tow it gently.
You should secure the chainplates before you take it anywhere, towing or otherwise. Out in the main channel, the wind will create enough motion and stress to pull the chainplates out even if you have no sails
up. Chainplate gives way and your mast is heading for the drink on the next wave. Your idea of running straps under the keel and to the other side is clever....if done properly it might work well. You might also think of fastening a 3/4" x 6" 48" long piece of lumber
to the outside hull parallel to the deck- hull seam but at least 5" below it using bolts through the hull every 6" and reinforcing with small blocks of wood on the interior
. Then connect the shroud
to that as a temporary external chainplate. ( you will need to attach multiple lines to the bolts and run them to the shroud
to distribute the stress) It won't be "sailable" but it should hold well enough for gentle motoring. Never underestimate the stresses from a 50' high mast wobbling around in choppy seas. Yes, you will have to do some fill and fix later, but at least the boat will survive.
Be sure to check the other chainplates as well. As you know, they are "hidden" behind some nice but deceiving woodwork. It's worth it to remove that and see what the real condition is. Accessing from the head
does not tell the whole story... check both sides.