Buco:
Not sure how I'm going to get the old one (and clean it up) of as it's really stuck on there.
That's as easy as 'walking in the rain', haha Imagine all the surfaces on a 35 years bike. I can say I've done them all, except case split, and the cylinder block, the head, etc., that would really bring the reality of rebuilding this bike into perspective.
The method I use is an exacto knife blade. With a control, if you can do it and you'd learn it with time, I shave off the old stuck and hardened gasket. Try different angles and cutting motion - you need to experiment what fits the scenario, as that would include different angles. First, longer strokes and very sharp angles. Then more scraping type motion and shorter more frequent strokes. With a feeling. Last would be some fine strokes with sand paper, 400 to 600, and polishing compound, carefully gated with cloth to prevent contamination. Protect all surfaces on the engine with cloth from scraps falling in, and whatever is taken off the engine can be rinsed in very hot water and dishwasher liquid soap to a perfect shape.
Years ago we use to rinse all in naphtha or kerosene and then blast it with hot water. It would dry and degrease all perfectly, leaving shiny aluminum.