A Plan
Looks like you have a handle on the big issues. The mounting block is probably the original and the correct size. If the engine is a four stroke, make sure the mounting bracket is specifically rated to handle a four stroke. Fours generate more torque than a two stroke and require a more robust mount. For the flexing you are seeing, check the interior wall of the transom for any obvious infiltration of water. Since the transom is usually not a cored area, delamination shouldn't be a problem. It may just need a bigger backing plate to handle the additional weight and torque of the fourstroke engine.For the deck, delamination is a common problem for any boat, especially one built as long ago as yours. The fact that it has been addressed is a good sign that the owner cares about his boat and maintenance. Bring a rubber headed mallet and give the deck some mild thumps. Hollow sounds and/or a squishy feel would indicate that the delamination has spread or wasn't completely fixed. Even if there is some delamination, it doesn't have to be a deal killer; unless it is widespread, minor areas of delam won't affect the seaworthiness of the boat for daysailing in protected waters. For the swing keel, make sure the board can be completely retracted and extended without jamming. Board jams are common. Sometimes its due to barnacles or other debris in the centerboard trunk, sometimes because the board has absorbed water and is now too fat to fit in the trunk, etc. Ask how old the pennant line is and when it and the shackle that attaches to the centerboard was last replaced. If possible, haul the boat and check the board for obvious problems.I haven't heard of rudder delamination being a particular problem, but it is always a possibility. A related issue is the condition of the rudder hardware. Oday used undersized pintles and gudgeons to attach the rudder to the transom. Check to see if they have been upgraded. If not, be careful of the condition of the existing hardware and consider upgrading.I also haven't heard of blisters being of particular concern on the Oday trailer sailer line. If you do find a significant blister problem, it will probably not be worth fixing as that can be a very expensive job. You would most likely be better off with a different boat.The rigging on these boats is simple and easy to inspect. Be particularly careful looking at the mainsheet attachment point on the backstay. It is the highest stress point on the rig and should be rust and corrosion free.You didn't specify, but I assume you will be doing this inspection yourself. I urge you to consider hiring a marine surveyor to do a pre-purchase inspection for you. Even though this is a simple boat, you will likely learn a great deal by following the inspector as he goes through his paces. I used one before buying my Oday 23, and his advice and insight were invaluable. Good luck.