I’m using Health Check but prefer CCleaner's original way of cleaning. How can I set that as my default cleaning method?

You can do this by setting Custom Clean as your home screen:

We changed the name of CCleaner's default cleaning method to Custom Clean due to the release of Health Check

How to set Custom Clean as your home screen:

Go to Options > Settings in the CCleaner app and change the CCleaner Home Screen to Custom Clean.