It all depends on what your plans are. If you're building from scratch and you have nothing but the engine, and the rest of the parts are readily available, then just build the bike with the right bits.
Either way, that engine you have is pretty much there, bare stator, crank and fly. The rest can be retrofitted.
If you just have a rolling chassis and you don't mind having a bit of a mongrel then fit the later parts (loom/CDI).
If you want it to be original Loom/CDI, then swap the crank/stator/flywheel over. Grab a kick starter, idle gear and lever.