You want to make sure the ties are solid such as alloy tubing.
If you use tensioned yacht rigging wire etc then it will keep the transom from flexing backwards but it will still flex forwards which kind of defeats the object.
Tie Bars
Use Stainless Steel Tube Diameter 20mm, Weld a left hand M12 Nut on one end and a Right hand M12 on the other.
Make 2 of these.
Buy 4 rose Joints available from RS 2 left hand and 2 right hand.
Make your brkts for the engine and the floor.
Drill the transom well at the correct angle then fit tie bars.
This will allow you to tighten or loosen as required.
Total price if you can weld yourself is about £55-60
Excluding Floor and engine mounting plates.
Hope this helps.
I have drawing of engine/ floor brackets for this application
Should you require then.