This is like someone asking a the question, "Why won't my tractor start?" without any factual info. The answer you get will be more conjecture that based on solid facts. Having said that, I'll venture an answer.
It depends on what the top of the block deck and the bottom of the cylinder head look like. f they are just dusty, wipe them off with mineral spirits check them for flatness, make sure the valves are good,and if good ,wipe out the bores and put it together. If they have light rust, plug the bores with rags, use a fine emery cloth and polish them, clean off any "rust dust", check for flatness and if good, reassemble. You might want to take the head in and get it checked out.
If you dismantle the whole engine you'll have to check all the bearing clearances, journals, etc and you're liable to get into some real $'s. for a little bit of normal wear and tear.Your dollars, your choice.