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Interfacing topological insulators (TI) with magnetic materials enables accessing quantum effects for advanced devices. The synthesis of such heterostructures faces challenges due to interlaying mixing and the often-complex multicomponent materials required to combine the desired properties. An alternative synthesis to direct growth is the modification of 2D materials by topotactic reactions to introduce new functionalities into pre-formed single or few-component 2D sheets. Here, the self-formation of bismuthene (a bilayer of Bi; Bi2) is utilized by thermal decomposition of Bi2Te3 as a platform to create novel 2D overlayers on the TI- substrate. Specifically, the Bi2 layer is modified by reacting it with vapor deposited Mn in an attempt to induce magnetic properties. Scanning tunneling microscopy indicates that the Mn modified surface remains in a planar 2D layer, indicating the formation of an ordered Mn-rich surface layer. Density functional theory (DFT) is used to develop models of both the Bi-rich Bi2Te3 surface as well as their subsequent Mn-modified structure. The DFT models indicate the possibility of magnetic ordering of the high spin Mn2+ ions. The successful synthesis of planar MnBi alloys on a van der Waals material illustrates topotactic reactions as an alternative for creating novel layered heterostructures.