Throughout my years of turning spanners ive ran into several degrees of difficulty in removing gaskets from mating surfaces.
Occasionally I get lucky and they come right off with no other work - more often than not it’s necessary to get a little more involved.
I tend to avoid razor blades, as they can sometimes do more harm than good, although I do keep one handy for the times i need it.
What I have found that works well, for really stuck on gaskets, is acetone or carb/brake cleaner to loosen the gasket a bit. remove as much as possible with a blunt blade or scraper, brass/copper/aluminum work well here as it they shouldn’t cause any harm to the harder cast mating surface.
Once you have the leftovers thin enough to your liking, grab an oil stone - this is a tip I picked up on in a few older dirt bike service manuals. I found a fine/rough coarsed knife sharpening stone at harbor freight for $5 that works very well on moped sized engine applications. Add a few drops of oil to the ‘fine’ side of the stone, and gently rub the remaining gasket away.
This technique is also great for removing imperfections in mating surfaces (the PO used a flathead to pull the clutch cover, see it all the time!)
The key is to keep your stone flat and when possible span the surface you are grazing to keep things uniform.
Your mating surfaces will come out looking factory and ready for install, with peace of mind that things will seal properly.
Lapping mating surfaces on glass or granite is also a swell option, but I’ve found the oil stone is nice to get into tight or hard to reach spots.