Suggestion:

Take the head off,
Blank off the exhaust port ( if 2-stroke) & carb intake, put the engine in very HOT WATER up to the top of the head so that the internals are dry,
Wait 10mins, & remove with gloves, & put cold WD-40 around liner / head.

It should slide out!

This works because the heat expands the case, and you can take stuck on backplates off, etc...

If you get water inside, you will need to remove it, use a hair-drier, and then back to the WD-40 = no corrosion later.

Good Luck

PS did you have a white Xmas?