The 5700, 6700 and 6800 all had OHV 2-cylinder vertical engines, which I believe are 18hp.
The 3600 is a horizontal, single cylinder 12hp engine. Different snowblowers were made for the larger engine models due to the vertical engines. I'm sure you could fab anything you wanted, engine-wise, into the compartment...I have not found a direct replacement anywhere.
Rattosh51
YT6800