bets thing to do is GET A REPAIR MANUAL
I purchased a GL1100 and fortunately for me, it came with a repair manual. Good thing because the first thing I had to do was replace the stator. FUN, FUN, but I learned a lot about the bike. They are easy to work on, just time consuming for a weekend mechanic.
If you can get it to turn, check compression. Hard to find a threaded compression tester. Might have to go to a bike service place for that. Threads are not the same as cars and I couldn't find one at any auto parts store. the plug type testers might be fine, but I find them to not be as accurate as the threaded ones. If both side and all four cylinders have about the same compression, you should be good. If not, you are likely just looking at a head gasket replacement. Fortunately Honda has a gasket maker that is WONDERFUL. They use it to actually seal the two halves of the motor together. I found some at a local bike repair shop and he just let me borrow some when I put my carbs back on.
Good luck with it and once you get it running, let us know.