I was talking to Steve Cooke who is a local Harley expert & restorer - he also owned a M3W for a while. He said:

"The mounting holes at the rear of the motor where the alternator lies are generic across the board. So basically you can put any Harley motor in. They go up to 117". The new M8 motor can go up to 134" with work. It all depends what you want to do. The faster you want to go, the less reliable the motor becomes."

He also recommended the S&S 124" twin cam injection which puts out some serious horsepower although he'd junk the fuel injection and fit a big Mikuni HSR48 carb!
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I guess any of these options would need new engine brackets and probably modified intake & exhaust manifolds. Quite a bit of work.

Andy