A few other things to try before you go backwards (and you really don't want to go backwards -- previous versions a quite a bit less stable than the current one.)
1: If you had Motion 4, you should delete the contents of
~User/Library/Application Support/Motion/Presets/Project folder
but only if you created custom presets
2: If you have corrupt fonts... you're in trouble with Motion. Use Font Book to verify all installed fonts and turn off/remove/delete any bad ones -- they'll have to be replaced with known good copies or do without. Motion 5.0.2 had some very severe problems with fonts and also with some of the content library shapes and/or option-click-drag duplications of Link parameter behaviors; so much so that if Motion crashed with those projects, NO version of motion will ever open them again! I had one of those -- I was working on a rather complicated video logo -- so very NOT HAPPY that happened. [If you have Autosave Vault enabled (Motion Preferences > Cache), you might be able to open one of the previous versions of those projects.. You might have to redo some "finish" work, but at least you'll have a really good starting point.]
3) delete preferences (use this: http://digitalrebellion.com/prefman/) [This is not likely to help -- but as a last resort before:
4) completely delete all copies of Motion 5 and reinstall from the App Store (you shouldn't be charged again). (If you have Motion 4 or earlier, these should be moved inside separate folders inside your Applications folder.)
You should be able to double click on those Motion 5.0.2 projects and have Motion 5.0.4 open them (unless those previously noted #2 exceptions are possible in those projects...)