HO hum pigs bum. It's my brother's computer and...well. I have no idea where the construction set is, it won't let me access it completely. Grhh this is very annoying! I might just have to get a quest complete or something...
Hmmm, this is going to be tricky then. Do you have access to Wrye Bash?
The Form ID for the body is xx02a868. The xx part will change depending on where Lost Spires is in your load order. If you have WB, you can right click on the Mods tab and select list mods. Go through the list until you find Lost Spires. Those two numbers in front indicate where that mod is loaded. In my game it's 39, so the id for the body is 3902a868. If you don't have WB, then you can count the mods to figure out where LS is loaded, but the two numbers are in hex, so you'd have to count in hex
Alright assuming you get the id for the body, load the game in the quest area of the fifth spire. Open the console and type the following:
prid xx02a868
moveto player 100, 0, 0
Replace the xx with the proper load order number for your game. Hopefully this will move the body to right behind where your character is standing. I'm counting on the body being considered an NPC, so this should work if the body was spawned but just isn't showing up where it should be. Here's a http://i855.photobucket.com/albums/ab118/AndalayBay/Oblivion/LostSpiresRuin.jpg of what the area looks like in the CS and where the body would normally be found - you might want to have a look just to make sure you're in the right location and we're talking about the right ruin
Give that a try and let me know how it goes!
EDIT: You could also get the proper form id, with your load order accounted for already, by using TES4Edit, if you have it, but that would give away some of the plot and I'm trying to save that for you