Blasting is the way to go, DRAIN THE OIL AND FLUIDS, flip it upside down and let it stand for the day, use plugs and duct tape to mask her up, take some time, get someone reliable to blast the motor.
Doesn't have to be flash media finish if you are painting it, fine garnet with nozzle not too close, rougher finish helps paint to stick.
Next grab your favourite engine paint, spray the lot, then finally to finish the project replace all the casing screws, head nuts etc with new ones from the shop. I buy those bolt kits and just replace one bolt at a time so not to disturb the gaskets. My MX250 was done that way.
http://ozvmx.com/community/index.php?topic=6756.0