For the initial hard yards clean, it might be worth getting the engine bay professionally steam cleaned. It's not expensive.
See Car & Truck Cleaning Services in the Yellow Pages for your nearest operator, or to book a mobile service like the used car yards. A good steam clean on a hoist before restoring a car, including the diff, makes the strip down a much more pleasant task.
Or most equipment hire places will rent out a Gerni pressure cleaner, but this can be a messy job with gunk flying everywhere.
Or if it's not too bad, then cans of Spray Degreaser from Supacheap/$2 shops etc will do the trick. Just don't drown the carbie or dizzy when you hose it off. A plastic bag tied around each is the go. After you spray on the degreaser, use an old paint brush to loosen the caked on stuff before giving it a good squirt with the hose.
Then after the initial major clean you can identify and fix any oil leaks so as to keep it that way.