The crack you are seeing is the collar for the fly wheel. It taper locks the flywheel in place on the crankshaft. No issues unless the flywheel came loose. I saw the engine make a couple of attempts to turn with the roto start so I think it is just the one-way bearing inside the back plate...
Welcome to RCT!
Getting into nitro at first can be very frustrating and has a fairly big learning curve. Hang in there, it's worth it in my opinion. I'm not on here as much as I used to be but I will try to help you out as much as I can. Now lets see if we decipher few of the issues.
First off...