Seriously though, that style of bike requires some serious futzing around to get almost any motor into it.
This is one of the reasons why a beach crusier style bike is the preferred frame to work with.
My Phantom 85 is about the same configuration, size wise, as the Avenger, and here is how it fit into my Hyper beach cruiser w/ 26" wheels...Fits like a glove with almost no futzing around to get it to fit in the required amount of space.
If a YD/LD/P-85 will fit, the Avenger should fit. They are all almost exactly the same in terms of dimensions. Any difference in height will be fairly minor.