The original analog circuitry
The MS-20 mini painstakingly replicates the original MS-20. A development team led by the original engineers themselves worked to recreate the original circuitry, and when it was necessary to substitute a part, these engineers made the decisions based on careful listening, in order to reproduce the original sound faithfully.
In fact, the sound of the MS-20 mini has a somewhat bright and extreme quality to it because its sound is that of an original MS-20 in mint condition at the time it went on sale, before any of the components aged.