The Sportster engine has a engine/transmission that is unit construction (all in one case). The FXD is 3 different cases (engine, transmission and primary) bolted together. The Sporty and FXD have completely different mounting points. The FXD is rubber mounted, Sporty's untill 2003 are rigid mount. You can get a hardtail frame that you can use your motor in (with special mounts), but your tranny and pimary would need to be changed to a softail version of each to work with it.