Brabus offers a V6 engine block for the Smart, with performance internals. The Triumph TR7 engine is 1/2 of the Stag V8 (as evidenced by the cross-flow head and location of the distributor). The latter was criticized as unreliable but British enthusiasts have further developed the engine and, with regular maintenance, runs reliably.
A V12 from the slant 6 would need cross-flow heads and the relocation of the distributor - effectively, a new block and heads.
