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I experienced the same problem as you with a dizzy on a high mileage utterly flogged 253 years ago. When I removed the rocker covers, oil poured out everywhere. There was so much sludge in the motor that the oil return galleries in the head were completely choked.
I couldn't believe the fight I had with the dizzy to get it out. No doubt, that's what the problem will be this time.
Only other alternative I can see is to remove front cover etc as discussed.
Hey, that's a good idea!!! there's about 2-3mm protruding from the block, so I could get my dremel, and grind a slot onto each side of the sleeve. Then I could get something long and flat, and use that to turn it!
You're a smart man, I'm adding that to my list of things to try!!!
Thanks for the brainstorm guys, this is so much more effective than my bolt + washer + rope + anvil + bash it down with a mallet trick