When buying a used bike from the years of manufacturing around 1995 to 1998, there is the question whether it is the older GS model with Rotax 123 or the newer MP model with Rotax 122. This is not clear by the years of manufacturing, as those models were partly sold in parallel. The MP model is available with the same look as the GS model, but the engines differ considerably.
Since also the frames have changed, you can easily tell the difference from the outside of the frame (as long as nothing has been rebuilt, which unfortunately you can never know for sure):
The frame from the MP-model on with the Rotax 122 has an additional strut (or “thick plating”) on the left side, which was not there on the GS-model.
When you see sales advertisement you might see by the pictures (if they are good enough) which engine should be installed.