Very doable if you can afford to spend 10/15 millions of $
Any rich guy could happily do it without problems...
Getting 50% would take a few million dollars, but rewriting many past blocks is
much more expensive.
Even if any of this does happen, it can all be manually straightened out after control is regained.
Well he should buy the hardware and then run it long enough to have a longer blockchain
Once this happens how can you fix this? The only way would be to organzie a separate network with the old blockchain and telling miners to mine on it until it finally become once more longer than the "bad one"
It can be done but it require 1)a backup of the old blockchain 2)enough miners that know what to do