Welcome to the Cub Family.
Big Dog basically said it all, folks have their preference for different products to free up a stuck engine. I was told that Dextron TF-3 works real well because of the wicking properties, but haven't tried it yet.
I did find a product that really amazed me though. I was trying to free up a toasted adjustable front axle for a wagon that I am building. I tried Kroil, WD-40, and the CaseIH penetrating oil. None of them did it, and I was patient. I was introduced to Solvo-Rust from Loctite. Has excellent wicking properties. I judiciously sprayed the axle 3 or 4 times a day (everytime I passed by it) and after 4 days, I repeat only 4 days, the axle broke free with a couple small wacks with a 3lb hammer, where it refused to budge before.
I am going to get some of this in a can other than a spray can, and I am going to use it to get Jethro unstuck. It is inexpensive and man does it work.
While I am at it, couple things that might be helpful to you.
At the top of this forum is a post called "Announcement: New Members and Visitors, Please READ". Have a read on it, follow the links to the ATIS FAQ's, the Best of H.L. Chauvin and of course the Manual Server. There is a lot of info there for anyone who is wishing to maintain, repair or restore their Cub.
Happy reading, and again welcome to the Cub Family