The oldest, deepest, and largest basin recognized on the lunar surface is the South Pole-Aitken (SPA) basin. In the time since its formation, several processes have modified the original interior of the basin, including the introduction of foreign material by impact basins during the period of heavy bombardment. These later basins redistributed material ballistically across the lunar surface forming a mixed and crushed zone on the scale of about one-kilometer deep. Models of crater excavation, ejected material transport, and mixing during emplacement are used to estimate the amount of foreign material from each basin event introduced into SPA and the degree to which that material mixes with the surface of the interior of SPA. We varied the size of the transient craters for all basins, the degree of mixing between foreign and local material, and the number of basins considered in our evaluation. Our modeling results indicate that materials derived from the original SPA melt breccia comprise at least 15% of the present regolith. The most realistic combinations of model parameters predict a SPA melt breccia component that ranges from 50--80% of the current surface regolith. The compositional character of the SPA interior has apparently not been obliterated by aeons of subsequent basin-forming events.