If you do decide to go through the engine that you have, this link is usually helpful.
http://www.rcramer.com/bikes/ex250/rebuild_notes.html
I've been down that road. Did a bunch of top end work. Spent more time and money than I should have. Engine swapping (and I even used a new gen engine, which added some extra work) was easier. If I hadn't been so damn lazy and busy, the swap would have been a single day event.
If you're familiar with the bike and prepared for the job, you can have it stripped and engine out within an hour or two.