Hot tanking would be the way to go if you have the equipment, if not most paints can be readily removed with caustic soda, cheap and readily available in the cleaning department of most DIY stores. Please wear gloves, this stuff dissolves paint it does the same to skin, hair and fingernails. Do not use a natural fiber paintbrush to apply, it will also dissolve. Do not allow caustic soda near aluminium or brass components.
Once you have softened/loosened the paint with caustic soda hit the casing with a pressure washer, this should remove around 90% of the finish. I like to clean off the last remnants of paint with a rotary wire brush, these come in all types and sizes to fit electric drills and angle grinders. They are also great for removing any rust.
Once you have it clean, put a coat of primer on it immediately, I can not tell you how disappointing it is prep a casing for paint only to find that it has had condensation form on it over night and the whole thing is covered in surface rust.
Please don`t paint it pink! several years ago I was painting a crankcase and was running out of red oxide primmer, I had a tin of grey primmer and decided to mix the two together, I didn`t dare post the photos as I would probably have got thrown off the forum for being a dangerous extremist, a homosexual or a communist or some combination of all three.
Brunswick green is the original color for a Lister engine but I have seen them looking good in many other colors. One of my favorites was a guy who did his engine with black lead stove polish and linseed oil, it looked as scary as Darth Vader.
Let us know how you get on.
Bob