As a direct bolt in from a Harley, it would need to be pre-'06 vintage. I believe ACE #10 has a Harley crate 120R "B" motor. S&S does NOT make a "B" motor for a Harley replacement, but do offer a complete line of Harley Twin Cam alternatives for touring bikes. The touring Harleys are NOT counterbalanced of that vintage and may not be a suitable replacement. From several articles, what Pete Larson ended up with was the "B" motor fully counterbalanced.
Unlike the ACE, which I have no idea of the interconnection between the motor/trans, the M3W has to live with what's there for the Centa housing adapter bolt pattern and that's the pre-'06 Harley pattern. Also the crankshafts on the newer Harleys are different. The pre-'06 have the shorter length of splines and an external "nut" type fastener, this is identical to the current S&S Wedge motors. The later Harleys have a full length splined shaft with an internal bolt fastener. Also on the later Harleys, the primary bolt pattern was upgraded to a 5-bolt pattern from the earlier "forever used" 4-bolt pattern that the S&S Wedge motor based its design on.
As Phil said, there's nothing that cannot be made to work once you find a suitable power plant. The later M-8 Harley touring motors, since '17 up, have a single counterbalance shaft. So even the touring models have "some" means to reduce vibration. The new Softail Harleys have the twin counterbalance shafts and would be even smoother. But again, the M-8 will NOT bolt up to the M3w housing adapter.
S&S makes their 124" Hot Setup for the earlier Twin Cam Harley motors with counterbalance. So even an '04 Softail 88" Harley motor with twin counterbalance shafts "could" be rebuilt to a 124" displacement and bolt into the M3W.
Without changing lots of items, the Harley pre-'06 motor looks to be the easiest to install. They would use a very similar oil tank configuration, exhaust would not be that difficult and all of the motors can be setup with a carb if that's preferable. Thunderheart makes a standalone ignition system that uses the crank pulse system and has many pre-programmed ignition curves to choose from.
I very much like the looks of the new BMW 1800cc flat twin, although not a prominent in stature as the upright V-Twin motors. Would be a slightly lower center of gravity as well.
I just hope that whatever happens in January '21 that the production of parts remains for you guys. Keep your fingers crossed...
