If you're handy with the old spanners, and you've got the gaskets you'll need to hand, you can expect to be able to undo everything, whip the heads off, change the head gaskets and reassemble everything in a single day. Assuming the heads aren't warped and can be reinstalled after a clean-up. That's if you don't encounter any unpleasant difficulties like for example stripping a thread in a head... Hope that helps! Go for it!