Their gen of origin does not matter afaik.
Should be able to breed them by giving the Johto Corsola an Everstone.
This. You need an everstone for any pokemon with a different regional form from the region you are breeding in.
For example if you wanna breed an Alolan Meowth in Galar, you need an everstone too.
If breeding with two johto corsola, have the female hold the everstone. If breeding with ditto, just have whatever joto corsola you stick in with it hold an everstone.
If you want the offspring to inherit HA, make sure the female johto corsola has it if breeding with 2 corsola. If breeding with ditto gender does not matter anymore.
The easiest way is to get one is to have Shield =p
You can get a johto HA corsola from the npc regina in IoA by trading her a galarian corsola. She's the grey shirt npc that pops up in random places.
According to your original post you already have one though?
On a side note: I can breed one if wanted I guess, but we should take that to PM or the trade corner =p