Actually, the ACE and Aero are VERY similar, and use the same frame, but the Spirit is actually DIFFERENT. Mount points for the forward controls are different, and the engine is slightly different, too, performance-wise. I know, because I have the Spirit 750, and have gotten ACE and Aero 750 parts from others, but they didn't fit.
They styles of the ACE and Spirit are quite different. The ACE and Aero have longer front and rear fenders, and the seat and fender rails, along with their mount points, are different than the Spirit, which has shorter fenders, and has a more of a low slung, "chopper" style look to it, with the almost straight drag handlebars versus the "buckhorn" or "pullback" handlebars on the ACE and Aero. I think the tanks are mostly the same, except for underneath, where the petcock and vent tube are mounted in different places, to fit the different frames, and the dash with speedometer mounted on the tank looks somewhat different between them, too.