I've got a 1982 MonArk 1652. My transom is in great shape and I'm no expert, but I'll tell you what I think from what I can see. Unless you can wield aluminum or have access to someone who can, leave the top bracing alone. That means the middle floor brace has to come out. In mine, I have two braces that are riveted in place - which means you can put it back yourself. You can see in your pics where the lower edge of the wood was. Cut you new wood to about the same height as the old one was. You don't want the wood to go all the way to the floor. If it does, it will be in the bilge, which means it will get wet more often and start to rot that much sooner.
Also in mine, the transom wood does not extend completely from side to side. Behind the side braces on either side, there's about a half inch gap between the edge of the wood and the side of the boat. For your width, measure the width of the corner braces, then cut your wood just slightly shorter than the width of one of the corner braces. For example, just thinking vertically, if the space for the transom is 64 inches wide and the width on one of the corner braces is one inch wide, you cut your transom wood just a hair under 63 inches wide (like 62 & 7/8'ths). Then, you can slide your transom wood behind one brace, lay it flat to the boat transom, then slide it back over to center it up. Then, just lift it straight up to fit behind the top lip. That will give you almost half an inch of wood to use to bolt the transom wood back into place. The most challenging part about the process will be taking the side angles into account.
You should be able to do the whole thing with a saw, a drill and a rivet gun. Just remember to take accurate measurements.
