Differences in motor:
Depending on the year of your bike the motor can be a 790 or an 865 cc. This won't matter much if you plan on going BB so just know that its one difference. For me the easiest way to know which is which is by color of the jugs. Grey are 790, black are 865.
Next difference in motors is the cranks. The America/Speedmaster/Scrambler is a 270 degree firing motor which is what gives them that "lump" idle that makes them more "cruiser".
Bonnies of all models and Thruxtons are 360 degree firing motors which makes them rev up faster (this is what I hear).
Both motors, aside from that crank, are mostly the same.
Keep in mind also that the 360 motors sit in a different frame and that means that things like the intake manifolds are different. The straight intake manifolds of the 360 motors won't work in the angled back backbone frames of the cruisers. You can reuse you intakes but that straight intake is part of what makes some of the power in the bonny motors vs the cruiser motors. Im sure its minimal but its part of it.
The bigger expense will be finding the right ignitor. The 270 ignitors will not work with the 360s due to firing degrees/timing.
From what I can tell motor mounts and such are the same. Side covers are different but are interchangeable (someone on there has done it).
If your motors out and the bottom end is still in good shape and you don't want to spend a ton of $$ the BB kit is the way to go. The BB with carbs, headwork and cams gets some great power gains. With tuning your knocking on the 100hp door. No one may answer but your knocking.
I see your in SoCal and are close enough to BC and TPUSA that it would be well worth going to both and tailing out your options. I don't know if BC has a shop where they do work but I know TPUSA does and Carlos is the guy to talk to to at TPUSA. I spoke to him a couple years ago and have a plan for my bike just not the money to follow through with it.
