You can use anti-freeze and a crock pot.

I use it to clean small RC engines.
They come out looking like new in about 8 hours.

Just drop the whole thing in and they come out looking like new.
It not only removes the carbon it really cleans the metal.
A day and a half and you would be done with no scrubbing.

You did save the old fluid? Thrift store and $5.00 gets you a crock pot.

Ralph

1991 Toyota 4x4 Standard Cab Pickup.
engnbldr cam and head.
Doug Thorley Headers
Home Made Cat Backs
30x9.5x15 BF Goodrich All-Terrain M/T KM2